REPORT ON THE ECHINOIDEA. 127 



It becomes quite a common feature in many of the genera of the Cassidulidse and the 

 like; but although it is only in such forms as Pygurus and its allies that there is a tendency 

 to form an abactinal anal beak covering the anal system, it is mainly among the Spatan- 

 goids that the actinal anal beak appears as a modification of the subanal plastron or an 

 indication of its presence in an exaggerated form such as we have it in Pourtalesia proper. 



In those Cassiduloid genera in which such a rudimentary actinal beak is formed, the 

 width of the test is generally greatest at the posterior extremity {Cassididus, Rhyncho- 

 pygus). An anal groove is indicated early among the Jurassic and' Cretaceous genera 

 in Pygaster, Clypeus, and Hyhoclypus, and less distinctly among the Discoideae and 

 Galeritidae, and in the latter there is a trace of an abactinal anal snout as in the modern 

 Echinocreins or Cystechinus, and Urechinus, while a true actinal groove, although indi- 

 cated in Dysaster and perhaps in Asterostoma where all the actinal ambulacra are sunken, 

 is well seen in the Ananchytidae and especially in Infulaster, which by its abnormal outline 

 recalls to us strikingly the Pourtalesian genus Echinocrepis. 



The absence of a fasciole in such genera as Echinocrep>is and CystecUinus, and the pre- 

 sence of a distinct anal fasciole in Pourtalesia and Urechinus, plainly shows that in the 

 Petal osticha the earliest fasciole to appear was probably a subanal one, as it seems to 

 exist in such genera as Urechi?ius in which the anal snout exists in a very rudimentary 

 form, and in which the subanal shield (PL XXX. fig. 19) is quite faintly indicated. 



The subanal fasciole of such Cretaceous genera as Cardiaster and Micrasler, in which 

 it first makes its appearance among the fossils, would seem to bear out this view. 



In Pourtalesia proper I have in the description of the species called attention to the 

 structure of the apical system, and shown that the four genital plates are with one exce]> 

 tion in the trivium; and this is well separated from the bivium by the intercalated apical 

 interambulacral plates. ■ In Cysteckinus the separation is ■difi'erent, two of the genital plates ' 

 (PI. XXXV. " fig. 5) are associated with the bivium and two with the trivium, and the 

 bivium and trivium are separated by the intercalated apical interambulacral plates. The 

 same is the case wdth Urechinus (PL XXX. figs. 16, 17). 



In Echinocrepis, however, as in Pourtcdesia rosea, the genital plates are contiguous, 

 and the bivium and trivium are not separated, while in Spatagocystis the apical system 

 is like that of Pourtalesia proper ; the genital plates are connected with the trivium 

 (PL XXVI.'* fig. 8), and that is separated from the bivium by the intercalated apical 

 interambulacral plates. 



The many differently shaped species of the genera of Pourtalesise take their outline 

 from the greater or less development of the difi'erent ambulacral and interambulacral 

 regions. "The high and short ambulacral and interambulacral coronal plates, nearly of 

 uniform size, of the anterior portion of the test, combined with a moderate elongation of 

 the posterior lateral ambulacra and interambulacra, give us such forms as Pourtalesia lagun- 

 cula, Pourtalesia Jeffrey si, and Pourtalesia miranda; with lower coronal plates in the 



