152 PALEONTOLOGY OF NEW YORK. 



the existence of these posterior scars in the Russian species and was incUned 

 to regard the specific difference in the Orbicula antiquissima and Pseudocrania 

 divaricata as not great. With Pholidops, therefore, they can agree only in the 

 fact of being unattached shells, the two distinct pairs of muscular impressions 

 on the inside and, in P. divaricata, the posterior beak and radiating ornamenta- 

 tion on the exterior, separating them definitively from this genus. The features 

 upon which McCoy proposed to separate these species from Crania should not 

 be given too great importance ; the unattached habit of the shells throughout 

 their existence may prove of value as a basis for a section of the genus Crania, 

 but the fact that the central muscular impressions are often larger than the 

 anterior, will not hold good for these forms only, as it is often seen in the 

 American species of Crania. We are disposed to agree with Mr. Davidson in 

 regarding Pseudocrania as synonymous with Crania. Pseudocrania divaricata 

 is from the Bala limestone and Llandeilo flags ; P. antiquissima from the Vagina- 

 ten-kalk, near St. Petersburg. 



EicHWALD,* in 1860, propose'd to designate the species, Orbicula antiquissima, 

 by the term Pal^ocrania, as it differs from Pseudocrania divaricata in the cen- 

 tral position of the beak and the concentric markings of the shell. This 

 proposal however is illegitimate, as 0. antiquissima was the first type of Pseudo- 

 crania and therefore is not available as the basis of another genus. 



Should these two species not p'ove congeneric either with each other or with 

 Crania, Eichwald's term will, under any circumstances, prove inadmissible. 

 When the Orbicula antiquissima becomes better known it may furnish a satisfac- 

 tory foundation for Pseudocrania as a genus.f 



* Lethaea Rossica, vol. i. p. 909. 



+ The only American species which has been refened to Pseddocrani* is the Crania {Pseudocrania) 

 aTiomala of A. WrscnELL, from the Hamilton group (Kept. Grand Traverse Reg-ion of Michigan, p. 92. 1S6C). 

 By the favor of Professor Wincukll we have been allowed to examine specimens agreeing with the original 

 description, and are compelled lo pronounce the name a misnomer, as the species is a well defined strepto- 

 rhynchoid. 



