Cretaceous and Tertiary Cirrij)edes. 189 



and upper latus arc of the same type as tlioso of the otlier 

 species of the <jenus, the scutum is quite unlike tliat of any 

 of them ; the tergum is not known, Iji the scutum there 

 is no promiucnt apico-basal ri(l<>e, and the growth-lines do 

 not differentiate the oechulent portion from the tergo-lateral 

 j)ortion as in the other species. In fact, if the ornamentation 

 had not been so strikiuiily similar to that of the other valves, 

 one would doubt its belonging to the siime species. Some 

 hundreds of separate valves of different species have been 

 obtained from the Chalk iNIarl of Cambridge, but none have 

 been found to agree in ornament except the above valves, so 

 the probability is that, despite the dift'erent form of the 

 scutum, all these valves belong to the same sj)ecies. In form 

 the scutum approaches that referred by Bosquet to his 

 Brachi/Iepas Utliotri/oides, but in that species the valve is 

 comparatively thick and massive, is quite ditfercnt in the 

 structure of its inner surface, and can readily be distinguished 

 by its flat and coarse longitudinal ridges. The rostrum and 

 upper latei'a are similar in form to those of P. ric/idus, 

 P. brilnnichi, and P. paronai, but can be distinguished by 

 the much more widely-spaced longitudinal ridges, aud the 

 spiuose appearance of the valves. 



Structure of the Species of Pycnolepas. 



In my paper on '' Brachylepas cretacea" (Geol. Mag. 191.2), 

 the species PoUicipes fallax, Darwin, which had been referred 

 by Dr. H. Woodward to his genus Braclvjlepas, was left out 

 of consideration. This was done chiefly because it seemed 

 probable that to whatever genus P. fallux, Darwin, belonged, 

 the species PoUicipes paronai, Alessandri, P. elec/ans, Darwin 

 { = P. briiimichi), and P. riyidus, J, de C. Sowerby, belonged 

 also. Au examination of the known valves of these three 

 species seemed to show that they were related in form, 

 structure, and disposition, and were precisely similar to the 

 corresponding valves in P. fallax. 



P. paronai, P. eleyans, and P. riyidus were represented by 

 carina*, scuta, aud terga, and if similarity in sha|,e and 

 structure were criteria, one would expect to find that these 

 three species had a huge rostrum and a long and narrow 

 upper laius, as in P.faVax. This conclusion, strengthened 

 by the fact that P. riyidus occurred in the Gault clay, made 

 it seem advisable to wash such material as could be obtained, 

 in the hope of finding the rostrum aud upper latus of 

 P. riyidus, and these valves were eventually found and 

 proved to be similar in shape to those of P. falla.c. Now 



