54 Dr. F. A. Bather — Studies in Edrioasteroidea. 



Measurements in millimetres are as follows : — 



a c g h 



Greatest length of turret 8-2 7 8-1 5-3 



Diameter at adoral end . 4-0x4-4 4-9 5-1x5-7 2-2x3-5 



Diameter at distal end .ca.3-0 3-2 ca.3-6 1-5x2-8 



Distinctions betiveen the Wenlockian Species. — In spite of the 

 variation as regards number and distinctness of vertical columns, 

 there are good reasons for regarding all the Coalbrookdale specimens 

 as belonging to a single species. First, the variations themselves 

 merge into one another, and do so in such a way as to suggest that 

 they represent stages of development, from fewer to more columns, 

 and from alternation to seriation of plates. Secondly, there is little 

 or no variation in the shape of the individual plates and in the nature of 

 their ornament ; and it is by characters such as these, little influenced 

 by the cruder forces of the environment, that genetic differences are 

 usually distinguished. 



If now we apply the conclusions drawn from these Shropshire 

 fossils to their contemporaries from Gotland, we find that the 

 differences relied on by Aurivillius to distinguish his species are for 

 the most part those which appear to us as simple growth variations. 

 So far as figures and descriptions allow one to judge, there seems no 

 good reason why S. sulcatum, S. varium, and S. fragile should not all 

 belong to a single species, for which the name Pyrgocystis sulcata would 

 most naturally be adopted. S. strobiloides, represented only by the 

 sole fragment found at Mulde, is probably no more than a slight 

 variation, owing much of its different appearance to difference of 

 preservation. S. granulatum seems to be distinguished by the 

 nature of its ornament, but this again may only be due to better 

 preservation ; we have seen in P. ansticei that no weight can be 

 attached to the difference in the direction of the plates at the distal 

 end. Of the two long, cylindrical forms, S. cylindricum, known only 

 from a single specimen, has plates of moi-e pointed shape, and, 

 since its horizon is possibly different, it may perhaps be left in its 

 independence. The other, S. procerum, may also be distinct, but 

 is more likely to consist merely of elongate individuals of 

 S. sulcatum. It must be borne in mind that none of these 

 specimens is really complete. 



So much for the supposed differences between these Gotland 

 forms. On the other hand, all, with the sole exception of the unique 

 S. cylindricum, have plates of the same shape, namely, with the 

 visible portion forming, in the words of Aurivillius, a segment of 

 a circle. This fact, while confirming the view that they represent 

 but a single species, sufficiently distinguishes them from the 

 Shropshire specimens, in which the outline is consistently parabolic. 



In the absence of any evidence to the contrary, such as might 

 conceivably have been furnished by the oral face, these Wenlockian 

 species must be referred to the genus Pyrgocystis. 



The Relationships of Pyrgocystis. — It is clear that Pyrgocystis is an 

 Edrioasteroid, but it is not clear whether it is to be referred to the 

 Agelacrinidse or the Edrioasteridse. We need not consider either 

 the Cyathocystidas or the Steganoblastidae in this connection. 



