SEA-URcniNS. 489 



is probably due to the presence of the periproct near the apex, 

 and has, in consequence, little direct phylogenetic meaning. 



Galeropijgns may be regarded as dittering from Fyyaster sens, 

 str. by a marked acceleration in the characters of its tuberculation 

 and peristome. The former feature is already in the uniform 

 condition, no definite order of appearance being traceable for 

 individual tubercle series. The peristome is quite small, and 

 slightly excentric anteriorly, with no visible adaptation for jaws. 

 I regard the genus as a primitive member of the ^>acleolites-gvon\i, 

 with all the characters of that group except the subpetaloid 

 ambulacra. As there is a marked tendency to develop this 

 feature even among the Holectypoida, it seems that its production 

 in the descendants of Galeropygas could be naturally postulated. 

 Owing to the stratigraphical appearance of Galeroj^yyus, 1 should 

 consider it an offshoot from the Diademoida that hardly, if at all, 

 progressed along the Holectypoid line of descent before developing 

 t^triking acceleration in all its characters except the periproct and 

 the ambulacra. It is interesting to find that the position of the 

 periproct remained more or less constantly primitive in the 

 majority of the Jurassic descendants of Galeropi/gus (e. g., Echino- 

 brissus and Clupens), although the ambulacra early began to show 

 elaboration. In the periproct feature, indeed, the Pygasteridae 

 show a greater acceleration than the Nucleolitida?, although the 

 Holectypoida, are, in most characters, a retarded group. 



Even if the afiinity between Galeropygas and Pygaster were to 

 be proved to be less close than I have indicated in the diagram, 

 the characters of the ambulacral plating would show that it was 

 derived, directly or indirectly, from a Diademoid ancestor. As 

 I interpret the relations of the genera at present, Galeropygus and 

 Pygaster stand together at the root of all the Irregular Echinoids, 

 in structure as well as in stratigraphical position. The subsequent 

 modifications of the Gahropygiis stock I have briefly outlined in 

 a recent paper (Hawkins, 66), and I hope to amplify that re- 

 adjustment of the classification of the " Cassidulida) " at some 

 future time. 



2. Pygaster, Co mil us, and the E c h i n o n e i d a?. 



Since its first recognition by Desmoulins (4), the genus I'yrina 

 has been the occasion of great confusion. The extraordinary 

 similarities that appear when it is compared with Conulus make 

 the generic position of species ascribed to them more ditficult to 

 determine than their specific distinctions. Such a form as 

 P. desmouUnsi, with its elongated ovoid ambitus, is easily dis- 

 tinguishable from a Conulus, the species of which are almost, 

 though rarely quite, circular in outline. To restrict the genus 

 Pyrlna to such elongated forms would, however, result in a very 

 unnatural grouping of the species, and, unless details of the 

 anatomy can be traced, the distinction of a I'oughly circular 

 Pyrina from a Connlns becomes almost impossible. Theoretically 



