192 ON THE ANATOMY AND AFFINITIES OF 
particulars, as to require careful consideration in relation to the forms. 
at present under consideration. These are the Dzastyd@ or cumoid 
Crustacea, 
The typical genera of this group were first described by Mr. H. 
Goodsir, and they have since been the subject of excellent essays by 
Kroyer and Spence Bate. 
Cuma Rathkii (Plate XVI. [Plate 27] figs. 17, 18) may be 
selected for description as a good representative of the family. The 
animal presents anteriorly a short and broad carapace, having its. 
outer and lateral edges rounded and sloping into a bifid, median, 
anterior, rostrum-like prolongation. The longitudinal fissure of this. 
process divides posteriorly on the carapace, so as to embrace the 
anterior part of a more convex median lobe, which represents the 
tergal region of the head. The bifid median prolongation is not, as 
it appears to be, a frontal rostrum, but it is formed by two lateral 
processes of the carapace, which come forward and are applied to one 
another in front of the head. 
The carapace is succeeded by five broad thoracic somites, and 
these by six, narrower and longer, abdominal somites and a telson. 
The sixth abdominal somite has styliform appendages, but in the 
female there are no other abdominal limbs, and in the male such as. 
exist are few and rudimentary. 
Of the appendages, the eye is median, sessile, and not easily made 
out in spirit specimens; there are a pair of antennules, a pair of 
antenne and of mandibles, two pairs of maxilla, and eight pairs of 
thoracic appendages ; there is a small labrum and a bifid metastoma ; 
but these parts have too little resemblance to any of the organs of 
Pterygotus to need description in this place. 
The most interesting feature about this crustacean in reference to: 
the present inquiry, however, is the ornamentation with which the 
body and many of the appendages are covered. The surface of the 
integument appears in many parts irregularly reticulated, but else- 
where the reticulations assume the form of a regular squamous. 
sculpture, singularly like that upon Prerygofws, but on a very much 
smaller scale. 
The general form of the body and the paucity of abdominal 
members, combined with the peculiar sculpture exhibited by the 
Diastylide, attracted my attention strongly when, on a former occa- 
sion,' I endeavoured to trace the affinities of Plerygotus. Following 
the line of inquiry thus suggested, I pointed out that in many 
? Quarterly Journal of Geological Science, vol. xii. 1855. 
