60 CEETACEOUS PELECYPODA 



to the Myidm, and when speaking of Necera, I also mentioned the true systematic 

 position of Theora; as regards Chamostrea I agree with Deshayes, that it is far 

 more prohably a variation of the Chama-iy\)Q of shells than that of the Anatinidm, 

 as restricted. 



Deshayes divides the family into three, Pandorid^, Osteodesmidm, and 

 FnoLADOMiiDAi. The first corresponds with our pandorin^, but includes also 

 Jlyochama and a few genera into which Pandora has been separated. The second 

 name includes the recent genera of the thraciin^ and anatinin^. The name 

 OsTEODESMiD^ Certainly is not well chosen, for, as I shall subsequently show, the 

 author himself hardly appears to admit the generic distinction of Osteodesma from 

 Lyomia. Eor Pholadomya Deshayes proposed a separate family, which seems to 

 me scarcely justified by the character of the shell and animal; the fossil species, 

 like Oonlomya, Pleuromya, Somomya and others, show an intimate relation between 

 Anatina and Pholadomya, between both of which there certainly can be much less 

 distinction pointed out, than between Thracia and Anatina, both of which the 

 same author places in his Osteodesmid^. 



A catalogue of the recent species of the Anatinid^ has been lately published 

 by Mr. T. A. Conrad, chiefly following the classification adopted in H. and A. 

 Adams' " Genera" {vide Am. Jour, of Conch., Vol. IV, pt. 5, Appendix p. 49, 

 1868). It excludes the PandojiidjE which are catalogued by Ph. Carpenter 

 in the same number of the Journal (page 69, &c.). The total number of recent 

 species of the whole family may be estimated at about liO. 



a. Sub-famili/,—PANDORIN.K 



The animals of the Pandora;, so far as they have been examined, offer several 

 relations to those of other typical Anatwidm. The siphons are only separated 

 towards their ends provided with fringed orifices ; the gills are single, the foot is 

 small, digitiform, and the mantle united nearly all round. The shell structure, 

 o-enerally noticed as one of the prominently distinctive characters, is by no means 

 very different from that of some Periplomce, Thracia;, and AnatincB. In these 

 the uppermore calcareous layer is generally preserved in small granules or points, 

 while in Pandora (but much less in Myodora) the upper layer, consisting of small 

 prismatic cells, is usually more compact and homogeneous ; the external layer is in 

 both cases pearly, and often of considerable thickness in Pandora and a few allied 

 o-enera. The cartilage is internal, usually covered by a small ossicle. In most 

 cases, no external ligament seems to exist ; it is rudimentary in a few species only. 

 The genera Ilyochama, Ilyodora, Pandora (with Kennerlia), Coelodon, and Clidio- 

 phora have to be placed in this sub-family. 



1. Myochama, Stutchbury, 1830. Shell thin, irregular, right valve larger 

 and attached ; hinge in each valve, with two diverging teeth like processes, in- 

 cluding a triangular cartilage-pit, furnished with a moveable ossicle ; a very thin 

 external ligament ; pallial sinus short and broad, 



