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Generic description:—Body not very convex. Pereion much broader than the cephalon, increasing regularly in breadth up to the fourth segment and then decreasing again.
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Pleon with last segment large and triangular, emarginate at apex. Last pair of pleopoda single-branched, consisting of a single broad squami-form plate.
This genus I have made for an Isopod of which I took several specimens at Timaru, and since then at Lyttelton Harbour. It will, I think, come nearest to Cassidina, Milne-Edwards; however, it does not resemble C. typa so much as it does C. latistylis, Dana,* the figure of which I have been able to see through the kindness of Professor J. von Haast. According to Mr. Miers, C. latistylis is the same as C. emarginata, Guérin-Ménev., and is found at Kerguelen's Island.††
From Cassidina, however, my genus differs in having the last pair of pleopoda unibranched. In Cassidina the outer branch is present, but is almost rudimentary, while the inner and basal one is large and broad; so that Cassidina appears to be truly intermediate between Scutuloidea and some genus such as Zuzara, which has the two branches equally developed. Scutuloidea maculata, sp. nov. Pl. I., fig. 1.
Head moderately large, transverse, about twice as broad as long, produced obtusely between the bases of the antennæ. First thoracic leg short and stout, second long and slender, the rest more like the first though not quite so stout, all having the propodos ending in two strongly curved claws. Segments of pereion subequal in length. Pleon of two segments, last large, triangular, with a wide shallow notch at apex. Last pair of pleopoda each consisting of a single broad squamiform plate, more than twice as long as broad, narrowing posteriorly, the inner edge conterminous with the side of the last segment of the pleon, and reaching very nearly to the end of pleon.
Colour—pale yellowish-brown, whole body thickly covered with small purple spots.
Length about ⅙ of an inch.
Hab. Timaru, among seaweed at north side of the breakwater; Lyttelton Harbour.
Additional remarks on structure:—
The eyes are moderately large and placed wide apart at the posterolateral angles of the head.
The upper antenna (fig. la) is considerably shorter than the lower; the three joints of the peduncle decrease in size distally and pass insensibly into the flagellum, which consists of but few joints. On the distal portion of it “sensory setæ” are found. These at first appear to be egg-cup shaped bodies, having a stout base from which arises all round a curved portion forming the cup. But careful focussing will show that there is
[Footnote] * U.S. Exploring Expedition, 1852, XIV., Crustacea, part II., 784; pl. 52, fig. 12.
[Footnote] † Trans. Royal Society, vol. 168 (extra volume), p. 204.
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another portion stretching out of the part already described, like a greatly elongated egg; this portion is exceedingly delicate and transparent; the small dot which marks the end of it is often more easily seen than the rest. (Fig. 1b.)
The mandible bears a three-jointed appendage; the first and second joints being equal in length and longer than the third; the last two bearing stout setæ which increase in length as they approach the distal ends of the joints on which they are situated (fig. 1c).
The first maxilla consists of two nearly straight lobes, the inner one tipped with slender plumose setæ, the outer one longer and larger and bearing strong serrated setæ at the extremity (fig. 1 d).
The second maxilla consists of three delicate overlapping plates; the two outer ones of which bear similar long simple setæ which appear to be transversely ribbed (fig. 1 f). On the third and inner lobe are setæ, two of which bear delicate filaments near the base only; the others bearing filaments on one side only throughout the whole length of the seta (fig. 1,e,f,g).
The maxillipedes have the basal portion long and straight, tipped at the end with several moderately strong setæ. This basal portion bears a four-jointed appendage, the joints of which decrease in size distally; the first three have the distal end produced into a rounded lobe tipped with setæ. (Fig. 1 h.)
The first pair of legs (fig. 1 k) is short and stout; the meros is short and expands greatly at the distal end, carpus very short, the dactylos is large and bears at the end two claws, the terminal one larger than the other which bears a small piece projecting on its inner side (fig. 1 l). The large claw appears to be more or less articulated to the rest of the dactylos. The second leg (fig. 1 m) is much longer and slenderer; the basos has its inner side fringed with short setæ, the meros is longer than in the first and expands distally, the carpus is slender and as long as the propodos; the dactylos ends with two claws (fig. 1 n), the smaller with several stiff projections along its inner edge, one towards the base of the claw being much stouter than the others. The remaining legs are somewhat like the first, though not so stout, being thus more or less intermediate in form between the first and the second.
The pleopoda or branchial plates have the basal joint broad and supporting two large branchial plates, the inner one being longer than the outer and broader at the base than at the end; both abundantly supplied with long plumose setæ (fig. 1 o). The pleopoda all rest in a cavity formed by the excavation of the under side of the segments of the pleon, much in the same way as in Sphæroma.