DESCRIPTIONS OF SPECIES. 433 



which it differs in not having its beak terminal, but set back some distance from 

 the rather obtuselj^ pointed anterior extremity. The beaks, however, are depressed 

 ■and scarcely distinct from the cardinal margin, and the general aspect of the shell 

 seems to be intermediate between that of Pinna and Avicula Modiola.^'' 



At the time of writing Meek was unacquainted with the constitution of the shell 

 in Aviculopinna, but later" he found that in the case of A. americana, a name- 

 which he proposed for Avicula pinniformis Geinitz (non. Geinitz, 1866), the very 

 thin substance of the shell had a prismatic structure "like that of other types of the 

 Aviculidse." Thei'e is, in fact, an inner nacreous layer covering at least part of 

 the shell, but this had probablj' disappeared, through leaching, in the specimens 

 examined by Meek. 



It seems generally to have escaped recognition that Hyatt* in 1892 referred 

 Pinna pei'acuta Shumard to Meek's genus Aviculopinna, and at the same time he 

 recoi'ded having observed both the prismatic and the nacreous laj^ers in that species. 

 The latter were found to be ''not present at the posteiior part of one specimen of 

 A. peracuta for at leat-t one-half of the entire length, as estimated." He was not, 

 however, able to study the distribution of the nacreous layer satisfactorily. 



One who I'eads Hyatt's brief discussion of the Pinnidee above referred to can 

 not but be struck by the closeness which Aviculoj>inna maintains to the nomencla- 

 turally older genus Pinna. 



The character chiefly relied on by Meek as differentiating Aviculopinna from 

 Pinna appears to have been the nonterminal position of the beaks of the latter genus 

 and the attendant condition of possessing a small anterior lobe. Hyatt remarks 

 regarding this feature in Pinna: 



"Unfortunately' the apex in all specimens of the Pinnidte is destroyed by attri- 

 tion. This is doubtless due to the habit of living partly buried in the sand. There 

 is therefore no way of proving that the umbones are absolutel}^ terminal, as has been 

 generalh' asserted in all descriptions. The umbones are doubtless more nearly ter- 

 minal than in ^4OT'CT<Zoj9mwa, but it is not safe to go beyond this assertion * * *."" 



The fact that while all recent shells have lost the beak and anterior lobe a 

 number of Carboniferous specimens have been found retaining them is significant, 

 and would appear cither to support H3'att's statement or else to indicate that 

 the habits of these Paleozoic shells were different from those of their living 

 representatives. 



As alreadj' noticed, Hyatt found the nacreous layer in Aviculopinna ? peracuta 

 to be absent for at least half the length, while in Pinna the fibrous layer alone often 

 occupies more than half the entire length.'*^ 



He describes the outer layer of the shell of Pinna as fibrous, while that of 

 Aviculopvnna is spoken of as prismatic, indicating a distinction which is more 



a Am. Jour. Sci., (2), vol. 44, 1867, p. 282. clbid., p. 337. 



b Proc. Boston Soc. Nat. Hist., vol. 25, 1892, p. 338. dibid., p. 336. 



W364— No. 16—03 28 



