Mr. H. G. Seeley on the Potion Sands. 27 



Not having seen Mr. Walker's specimens, I am unable to 

 speak with confidence on the other species named ; but no such 

 shells as Exogijra conica, Modiola cequalis, and Myacites jilicata 

 have come under my notice, though I have long had other spe- 

 cies of those genera in the Woodwardian Museum. 



X. The author's list of fish and reptiles needs but brief com- 

 ment_, the names being in part identical with those which have 

 for years been attached to similar fossils in the Woodwardian 

 Museum; but it can hardly be necessary to assure any one that 

 the genera Pycnodus, Hyhudus, Lepidotus, Gyrodus, &c. are 

 just as little found only in the Kimmeridge Clay as are the spe- 

 cies Asteracanihus oimatissimus and Lepidotus (Sphcerodus) gigas, 

 and that thei-e can be no reason for thinking them other than 

 tenants of the sea of the time. Had the author availed himself 

 more fully of the collections to which he appears to have had 

 access, he might have chronicled a more wonderful series of 

 fossils than those enumerated — a series as rich perhaps in 

 genera and species of fossil reptiles as any known geological 

 fauna, 



XI. The author quotes the existence, in the Woodwardian 

 M useum, of shelly limestone containing Cyrena, and uses this as 

 evidence for inferring some of the fossils to have been derived 

 from the Wealden. I can confidently say that no such speci- 

 mens have ever been found; and the concretions which were 

 supposed to be the said shelly limestone, on being broken, are 

 found full of Cardium, Cytherea, &c. Moreover I have shown, 

 in my paper on these beds, that the material of the deposit 

 came from the east. 



XII. Finally, Mr. Walker has described and figured (pi. 13) 

 two shells. The one referred to Sphara Sedgwickii is not a 

 Sphcera, but a Cyprina, and only differs as a variety from C. 

 angulata (Sow.), a type prolific in varieties. The form figured 

 is not typical. The species referred to Pliolas Dallasii may be 

 new. As every one is aware, all the secondary Pholades belong 

 to the genus Plioladidea. This species burrows in wood, and 

 lines its burrow with shell, and rather approximates to Xylophaga 

 and Teredina than to Pholas. It has no affinity to D'Orbigny's 

 P. Cornueliana. 



The age of the beds to which Mr, Walker's paper relates is a 

 difficult problem, and not one that can be solved by an appeal 

 to fossils, or mineral character, or superposition. And it is in- 

 timately bound up with questions of great interest, such as the 

 age of the Farringdon beds and the nature of the marine equi- 

 valents of the Purbeck and Wealden strata. For I have found 

 to the north of Cambridge most of the Farringdon fossils in a 



