155 



NOTE ON A RARE FORM OF ACTINO= 

 CAMAX {A. OROSSOUVREI) FROiVl 

 THE CHALK OF YORKSHIRE. 



(plate XVI.) 



G. C. CRICK, F.G.S. 



British Museum (Natural History). 



The interesting fossil which forms the subject of the present 

 note belong-s to Mr. J. R. Mortimer, of Driffield, Yorkshire, to 

 whom I am greatly indebted for the loan of the specimen. My 

 thanks are also due to Mr. Sherborn for calling- my attention to 

 it. Mr. Mortimer says the fossil was collected in 1863 close to 

 Fimber, in Yorkshire, from the flinty chalk. This flinty chalk 

 is assigned to the base ot the Micraster cof-angicimim zone by 

 Dr. Rowe,* who records from it, besides other fossils, three 

 ■examples of Actinocamax granidatiis and an undoubted speci- 

 men of Actinocamax veriis. 



The fossil under consideration is undoubtedly referable to 

 the species which M. Janet t described in 1891 as Actinocamax 

 Grossouvrei. The following is a summary of M. Janet's descrip- 

 tion, which was based upon three specimens, from the Cretaceous 

 rocks of France, exhibiting different stages of growth. 



The guard at an advanced stage of development is rather 

 massive and much depressed, having, when viewed from a 

 ventral aspect, its point of greatest width at about three-fourths 

 of its length from the alveolar end ; from the point of greatest 

 width it tapers gradually and regularly up to the alveolar 

 extremity, whilst in the opposite direction it tapers much more 

 rapidly and forms an ovoidal point. 



In the very young stages of development the transverse 

 section is, as shown by fractures of the guard, quite circular, 

 but with growth this section assumes a more and more flattened 

 form. A medium-sized specimen presents in a ventral aspect a 

 long and fusiform outline ; its point of greatest width is 

 situated, as in an example at a more advanced stage of develop- 

 ment, at about three-fourths of its length from the alveolar 

 extremity, but from this point it decreases much more rapidly 

 than in the older individual up to the level of the ovisac, | where 



* Proc. Geol. Assoc, vol. xviii., part 4, Feb. 1904, pp. 251, 252. 



t Bull. Soc. g'^ol. France, st§r. 3, torn, xix.. No. 9, Nov. 1891, pp. 716- 

 719, pi. xiv., fig-s. I, 2, 3, and text-figs. 2 and 3. 



X The ovisac is the small globular body at the apex of the phragmocone, 

 and is therefore situated at the bottom of the alveolar cavity. 



KJ06 May I. 



