382 CRETACEOUS PELECYPODA 



appear, however, iu the Trias, and their number increases gradually up to the 

 present date, when they reach their maximum of development. 



I have here accepted Philippi's suggestion of separating Pmwa into a dis- 

 tinct family in preference to classifying it with the Aviculidje. As regards the 

 disunited mantle-margins Pinna resembles Avicula, but the [comparatively strong 

 development of the anterior adductor, the origin of the gills between the palps, 

 the development of the median pedal muscles, and the rounded shape of the 

 posterior adductor, agree with the IIytilidm. Again, the shell of a Finna resem- 

 bles a very oblique Avicula, in which the anterior ear becomes obsolete, but the 

 manner iu Avhich the ligament is attached to the dorsal edge, and its great length, 

 a^ree much more closely with that of ILjtilns and Modiola, than with Avicula. 

 Thus the organisation of the animals, as well as most of the characters of the 

 shells, appear to me to point to a position intermediate between the former and the 



next family. 



1. Aviculopinna, Meek, 1864, (Am. Journ. Sc. and Arts, xxxvii, p. 212; 

 Geinitz, Dyas, p. 77). Very elongately sub-trigonal, equivalve, with slightly 

 indicated sub-terminal beaks, the shell being somewhat produced in front of them, 

 posteriorly gaping; hinge line very long, edentulous; type, Avic. jn'ifica, Miinst., 

 from Permian rocks. 



Judsins from Mlinster's figure in Goldfuss' Petr. Germ, the shell of this 

 species appears to be of appreciable thickness, but no account refers to the struc- 

 ture of the shell itself. Its general sub-trigonal form and posterior gape has a 

 very decided relation to Pinna, but the beaks are not quite terminal, and the 

 anterior end has almost more the form of a Ilodiola than of an Avicula, to which 

 latter genus Geinitz referred the species under the name of A. pi^incBformis. If 

 the posterior end were not gaping, but closed, the shell would be better classified 

 in the Mytilidm, as suggested by Meek. It also shows a close resemblance to 

 M' Coy's Pterinites, but this is not gaping posteriorly. 



2. Trichites, Plott, 1676, (Morris and Lycett, Moll. Great Oolite, pt. ii, 

 p. 32; Pinnigena, de Luc). Shell generally u-regularly sub-qviadrate, very thick, 

 fibrous throughout, inequilateral, and mostly inequivalve, often with prominent 

 more or less twisted and terminally obtuse beaks, slightly gaping in front of them ; 

 left valve larger, more convex and thicker than the right one, both with variable 

 rugosities on the surface ; hinge linear, edentulous, posterior muscular scar sub- 

 central, large, anterior very probably situated at the beaks, as in Pinna ; tj-jie, 

 Trick, nodosns, Lycett, from the great Oolite of England. Most of the species 

 known are from Jurassic deposits, only one has as yet been recorded from the 

 cretaceous. 



3. Pinna, Linn^ 1758. Sub-trigonal, with terminal, or nearly terminal, 

 produced beaks, the margin being generally slightly indented bcloAv the same 

 for the passage of the foot and byssus ; posteriorly widely gaping; hinge line 

 straight, edentulous, with a long almost internal ligament, anterior muscular 

 impression small, situated in the beaks, which are often corroded, posterior muscular 



