Arhynchobatis is a monospecific genus of skates based on A. asperrimus, a moderately deep-water species, endemic to New Zealand. It is a typical member of the Rajidae, differing from other genera in the absence of a second dorsal, and the presence of a complete caudal fin. It has been included recently in the Platyrhinidae, but the endoskeleton is definitely that of the g. Raja.

Waite (1909) established Arhynchobatis as a new genus to contain A. asperrimus, a new species based on a single female specimen 640 mm. in total length and trawled from 66 to 94 fathoms in the Bay of Plenty during the operations of the New Zealand Government Trawling Expedition of 1907. From the literature there has been no other record of the species, but Dr. G. A. Archey has advised me that a dried specimen was found in the Auckland area. This specimen, however, no longer appears to be available. The study material on which the present paper is based comprises two female specimens of A. asperrimus, comparable in size to Waite's specimen, trawled from 50 to 60 fathoms, south of Castlepoint, in September, 1953, by the steam trawlers “Maimai” and “Thomas Currell”. The first of these specimens taken (i. e., the “Maimai” specimen) formed the basis for the following systematic description, and later was dissected on one side so that an account of the skeleton could be prepared. The second specimen taken (i.e., the “Thomas Currell” specimen) was checked against the systematic description of the first, and the variations are included in the description given here.

In his original account, Waite apparently recognised his new genus as belonging to the Rajidae, as he refers to no other family, and considered his specimen as having affinities with Psammobatis in the absence of “a cartilaginous rostral and the character of the nasal valves,” but differing in having a distinct caudal and only one dorsal.

Garman (1913) included Waite's account in his monograph on the Plagiostomia, and places the genus in the family Discobatidae, where it is allied with Discobatus, Platyrhinoidis and Zanobatus. All of these genera are uniform in possessing a rostral cartilage and two dorsals, thus differing from Waite's account of Arhynchobatis where it is stated there is no rostral cartilage and only one dorsal fin.

Whitley (1940) includes Arhynchobatis in the Rajidae, and believes that it is allied with the Australian Pavoraja nitida, though he has not been able to examine any specimens of the former.

Fowler (1941), following Garman, includes Arhynchobatis in the Platy-rhinidae, a family which also includes Zanobatus and Platyrhina (= Discobatus). In a recent review, Richardson and Garrick (1953) having no material available for examination, followed the opinion of Garman and Fowler.

Garman (1913) gives an account of the significant morphology of Platyrhina (= Discobatus) sinensis. His figures (Pl. 66) of an entire specimen 25 ¼ inches in length, is that of an animal generally similar to Trygonorrhina or other of she fiddler rays, in other words having the entire disc longer than broad but rather sharply rounded anteriorly, and with the greatest diameter at two-thirds the distance from the snout to the posterior end of the pectoral disc; the tail at its origin is wider than the interspiracular width, and heavy throughout its length. In contrast, Arhynchobatis has the length of the entire disc subequal to its width, with the greater diameter midway between the snout and the hind margin of the pectoral disc. The tail is narrow at its origin, less than the interspiracular width, and slender throughout its length. These characters, which delineate the general facies of Arhynchobatis, are definitely those of the typical Rajidae and not of the Platyrhinidae (though Arhynchobatis has more rounded lateral angles to the disc than is usual in rajids). The only external diagnostic character which resembles that of the Platyrhinidae is the completeness of the caudal fin. But in Platyrhina the caudal is generally heavy and extensive, while in Arhynchobatis it is small and generally slender, despite its completeness. It would appear, then, that from the external characters, Arhynchobatis should be placed in the Rajidae as originally proposed by Waite. Such a proposition is further substantiated by the nature of the skeleton, which will be discussed later. Waite's suggestion that Arhynchobatis appears to be allied to Psammobatis due to the similarity of the nasal valves and the absence of the rostral cartilage is valid at least for the former character, though the generally similar nature of the nasal valves of all the Rajidae precludes these from being very reliable characters. The rostral cartilage (which is absent in Psammobatis, the anterior face of the mesethmoid being concave) is present in Arhynchobatis as a short, shovel-shaped rostrum, as seen by dissection of the “Maimai” specimen. Such a condition has no parallel in the other Rajidae as known to me, though there is sufficient variation in their rostra to allow Arhynchobatis to be included with them in the family.

The absence of a second dorsal fin in Arhynchobatis makes it unique among the Rajidae. For this reason alone, and despite the obvious similarities between Arhynchobatis and other rajids such as Pavoraja nitida as referred to by Whitley (1940), the distinctness of the species at the generic level is valid. This is confirmed by the presence of the complete caudal fin, a condition not occurring in other rajids. The generic diagnosis given by Waite does, however, need revision, in that the presence of the short rostral cartilage, and the nature of the posterior nasal valve, which can barely be described as triangular, should be noted. The amended generic diagnosis follows: