![Thumbnail: [Volume 82, 1954-55 page 130]](/journals/rsnz/images/rsnz_82/thumbnails/rsnz_82_01_0155_0130_ac_02.gif)
Summary
From the foregoing description of the skeleton of Arhynchobatis asperrimus it can be seen that the affinities of this species are with the Rajidae. The structure of the rostrum conforms to the type present in the Rajidae, although it does not agree completely with that of any of the rajid genera known to me; and although it agrees with the type of rostrum occurring in Platyrhina sinensis, that in itself is by no means enough evidence to retain the species in the Platyrhinidae. The median accessory rostral cartilage appears to be a novelty for the Rajidae, but if it is a chondrified ligament, as it appears to be, then its presence offers no obstacle to placing Arhynchobatis in this family. The antorbital supporting cartilages and the cartilages of the nasal valves agree closely with those of the Rajidae, the latter in particular offering valuable evidence on affinities considering the amount of variation shown by these cartilages in the various families. The jaws, hyomandibula and spiracular cartilage are not of great diagnostic value at the family level, but the remainder of the hyoid arch, the pseudohyoids, ceratohyal and hyoid copula are certainly indicative of the Rajidae, as are the branchial arches and in particular the ventral branchial elements. The vertebral column, though showing the characteristic features of all batoids in being fused anteriorly and bearing processes for the articulation of the pectoral girdle, is modified like those of other members of the Rajidae in that the fusion extends well behind the level of the pectoral girdle, and the pectoral processes are dorsolateral bars which are laterally very extensive. The latter affect the pectoral girdle, making it in turn typically rajid. The basalia and radialia are diagnostic only in that they separate Arhynchobatis from such highly specialised families as the Pristidae, Torpedinidae, Mobulidae, etc., but they do not separate it from the more closely related families with anterior terminal or quasi-terminal pectoral fins.
The pelvic girdle similarly is at once distinct from those of families with forward arched bars and median prepelvic processes (Dasyatidae, Myliobatidae and Mobulidae). But even in the remaining families with transverse pelvic bars, there are none apart from the Rajidae with similarly elongate, pointed lateral prepelvic processes such as are present in Arhynchobatis.
The skeletal distinctions between Arhynchobatis and the g. Raja are, with the exception of that of the median accessory rostral cartilage, almost entirely quantitative (i.e., differences in relative lengths and proportions of cartilages), or else where there are minor qualitative distinctions, homologues of these may be found
![Thumbnail: [Volume 82, 1954-55 page 131]](/journals/rsnz/images/rsnz_82/thumbnails/rsnz_82_01_0156_0131_ac_02.gif)
in other genera. [E.g., the large median hypo-basibranchial plate of Raja erinacea is shown by Garman (Pl. 68, Fig. 1) to be divided into an anterior and posterior portion, whereas in Arhynchobatis and Psammobatis mira (Garman, Pl. 69, Fig. 2) it is complete.]
It would appear then that Arhynchobatis may be regarded as a typical rajid, with only very minor skeletal distinctions separating it from the genotype of the family, and from the other genera.
The distinctions between Arhynchobatis and the Platyrhinidae (as exemplified by Platyrhina sinensis) are much more marked. The rostra are certainly similar, though Arhynchobatis has a median accessory rostral cartilage, and Platyrhina has lateral, divaricated accessory rostrals. The plate-like antorbital supporting cartilages and the divarications of the nasal valve cartilages of Platyrhina contrast strongly with the short, rod-like antorbitals and the nasal valve cartilages as described previously of Arhynchobatis. Similarly, the sinuous hyoid copula, the subrectangular median hypo-basibranchial cartilage, and the divided hypobranchials of Platyrhina differ markedly from the smoothly arched hyoid copula with prominent lateral processes, the subcircular median hypobasibranchial cartilage, and the fused hypobranchials of Arhynchobatis. But the most striking difference between the two skeletons is in the pelvic girdle. In Platyrhina the pelvic bar is very wide, slightly arched forward medially, and lacks distinct lateral prepelvic processes. In Arhynchobatis the pelvic bar is relatively much narrower, is almost transverse, and bears the characteristic rajid lateral prepelvic processes, which extend well forward as elongate pointed cartilages.
Such differences are obviously too fundamental to allow Arhynchobatis to be included in the Platyrhinidae, especially as the similarities that do occur are no more than one would expect between two members of such a morphologically and hence systematically compact and specialised group as the Batoidei.