26 BRITISH FOSSILS. 



Systematic Position of Pterygotus. In comparing Pterygotus 

 with other crustaceans, for the purpose of determining its systematic 

 position and relations, several of the largest and best denned orders 

 of the Crustacea may at once be left out of consideration. Ptery- 

 gotus is clearly not one of the Trilobita, Cirripedia, Ostracoda, 

 Edriophthalmia, or Stomapoda (if we restrict this order to these 

 Crustacea which have pedunculate eyes, and distinct and moveable 

 ophthalmic and antennulary somites). Nor are there any known 

 Branchiopoda to which these great extinct crustaceans have relations 

 of affinity, so that if they are to be referred to any existing order, it 

 must be either that of the Podophthalmia, that of the Copepoda, or 

 that of the Pcecilopoda. 



The first step towards determining the systematic place of 

 Pterygotus, therefore, is to consider the reasons for and against the 

 assignment of a position in either of these orders to them. 



To those who are acquainted only with the ordinary Podoph- 

 thalmia, the discussion of the affinities of Pterygotus, with forms in 

 every way so distinct, may seem superfluous ; and there is assuredly 

 little enough in common between a crab or a shrimp and their palaeo- 

 zoic congeners ; but there are one or two sections of the Podophthal- 

 mia which, while they still hold fast by the typical characteristics of 

 their order, become so modified in many important particulars, as to 

 require careful consideration in relation to the forms at present under 

 consideration. These are the Diastylidce or cumoid Crustacea. 



The typical genera of this group were first described by Mr. H. 

 Goodsir, and they have since been the subjects of excellent essays 

 by Kroyer and Spence Bate. 



Cuma Rathkii (Plate XVI. 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 



