FROM THE CORAL RAG. 321 



inter-ambulacrum is convex and prominent, more especially so nearest the border 

 (fig. 2 d). 



Affinities and differences. — This species very much resembles Collyrites elliptica, 

 Lamk. From the Great Oolite and Kellovian strata of the Department of the Sarthe I 

 possess for comparison a very good series of this species, collected and sent me by my 

 friend, M. Triger, besides other good types from MM. Michelin, Bouchard, and Cotteau. 

 I find that in Collyrites bicordata the shell is more cordiform, and the anteal sulcus is well 

 developed, whilst in C. elliptica that depression is absent. The left postero-lateral 

 ambulacrum generally rises higher up on the dorsum than the right postero-lateral 

 ambulacrum. The anterior summit is rather more forward, and the basal portion of the 

 single iuter-ambulacrum is more prominent than in C. elliptica ; whilst in the relative 

 position of the mouth and vent, and the distance of the apices of the posterior ambulacra 

 from the latter opening, there is much similarity between these two species. 



Locality and Stratigrapldcal position. — The Yorkshire specimens have been collected 

 from the lower calcareous grit, near Scarborough ; they are in general denuded of their 

 test, and are mostly deformed. C. W. Strickland, Esq., has collected several specimens in 

 the Coralline Oolite of Hildenley, where they are associated with Cidaris jlorigemma, Phil., 

 Heniicidaris intermedia, Flem., and Pseudo-diadema hemisphai'icum, Agas. James Carter? 

 Esq., of Cambridge, has collected this species from blocks of Oolitic drift at Holywell, 

 St. Ives, the rock containing these specimens is a ferruginous, coarse-grained oolite, which 

 I suppose may have been derived from the base of the Coral Rag, or lower calcareous grit. 

 This gentleman has likewise obtained fine specimens of the same species at Ely, from a 

 rock which he conjectures to be Kimmeridge clay. One of these urchins is figured in PI. 

 XXHI, fig. 2. It is unfortunate that a doubt should still exist relative to the true age of 

 these beds, and I feel under many obligations to Mr. Carter, which it afibrds me great 

 pleasure to acknowledge, for the series of specimens of C. hicordafus he has kindly 

 communicated and generously given me. Professor Sedgwick has likewise been most 

 obliging in making an examination of one of these specimens, and in expressing his opinion 

 relative to the age of the Oolitic drift at Holywell, containing C. bicordatus. I have much 

 satisfaction in adding that distinguished geologist^s opinion on the subject, which accords 

 with my own conjectures made from a paljeontological and not from a stratigraphical point 

 of view : 



" I have no doubt," says Professor Sedgwick, in a letter to me on the subject, 

 "the specimen is what is commonly called our glacial drift, not, however, a drift 

 brought on icebergs, but a drift caused by a great change of level about the end of 

 the so-called glacial period, and it contains fragments innumerable of rocks belonging 

 to the whole series, from the lias to the chalk inclusive. Our general order of 

 super-position round about Cambridge is, \?,\,,yravel and drift of different ages, irregular 



