BIVALVIA. 265 



surface, which passed in my ' Catalogue' under the name of A. asperima ; they are too 

 imperfect for further notice. The siphonal side appears too short to belong to 

 C. 2^TCEtenue, and too rugose for the present species. 



Pholadomya, G. Soioerby, 1823. 



Caedicm (sp.) Mantell. J. Sowerby. 

 LuTEARIA (sp ) J. Sowerby. 

 Cakdita (sp.) /. Sowerby. 



Generic Character. Shell very thin, transparent or hyaline, of a nacreous texture, 

 transverse, ovate or cordiform, ventricose, equivalved, inequilateral ; anterior side 

 short, posterior produced and gaping. Hinge with a small obtuse tooth. Ligament 

 external. Mantle-mark deeply sinuated. 



Animal of the form of the shell, with the edges of the mantle united, except where 

 open for the emission of the foot, which is bifurcated. Siphonal tubes large. 



Only one species of this genus is known in the living state, and that is an inhabi- 

 tant of the tropics and was found at St. Lucia.* The animal of this has been examined 

 by Professor Owen, whose observations thereon were made known at the Zoological 

 Society in 1842. 



Its position, as indicated by the animal, is considered by that anatomist to be near 

 to Panopcea. Dr. Gray, in his arrangement, has placed it between Cardifa and Astarte. 



It is, no doubt, very nearly related to a group of shells largely developed in the 

 Secondary Formations, for which M. Agassiz proposed to establish a family under the 

 name Myadce, 'Etudes Critiques sur les Mollusques fossiles.' These he separated into 

 several genera, the divisions depending sometimes upon the hinge furniture, but in 

 most instances upon the outward form of the shell, a dependence by no means safe.f 

 These fossils are generally found in a state of casts only, though some few of them 

 have been obtained exhibiting portions of the shell attached, showing them to have 

 possessed a thin, oftentimes a papyraceous and transparent shell, of a nacreous texture, 

 and they were in most instances covered with a papillaceous or scabrous exterior. They 

 bear a considerable inter-resemblance in their general character, and are no doubt 

 intimately connected zoologically ; but they are of very doubtful relationship to Mya, 

 the reputed father of the family, whose age we are unable to date beyond the 

 Tertiaries. 



* Ph. crispa and PA. caspica, given by Agassiz in his 'Monog. of the Mj'adse ' as existing species, 

 belong to, or at least are nearly related to, the genus Cardium, and differ only in having elongated siphons, 

 and a sinuated mantle-mark {Adacna, Eichwald). Lyonsia navicula, Reeves, 'Voy. of the Samarang,' 

 p. 38, pi. 23, fig. 1 1, may perhaps be an aberrant form of this genus. 



•j- This family has been ably analysed by Professor Morris in his recent 'Descriptions of the Fossils of 

 the Great Oolite.' 



35 



