62 CEETACEOUS PELECYPODA 



often with two teeth ; ossicle present, pallial line simple. All the species at pre- 

 sent known are from North- American seas. 



Pandorina oi Scacchi has been considered as identical with Lyomia. Tan- 

 dorella of Conrad is stated by its anthor to be a true Fandora (see Am. Jour, of 

 Conch., 1867, III, p. 269). 



Thus we have in place of the genus Fandora, as recorded in the works of H. and 

 A.Adams, Chenu, or Deshayes, three, and probably four, tolerably distinct 

 generic forms. 



In fossil species the determination becomes, however, very difficult, and in most 

 cases it only depends upon the external shape of the shell. It is, therefore, only 

 natural that, not knowing the hinge-teeth, the oldest name (Fandora) will 

 for some time be the safest to be applied. There are, however, a few fossil species 

 from tertiary beds which, if not identical with recent forms, allow a correct generic 

 determination. Fand, incequivalvis occurs in the miocene beds of Vienna ; some 

 others from the English tertiary beds mostly also belong to recent forms. Of the 

 species described by Deshayes in his second edition of the Paris fossils, Fand, 

 Defrancei, appears to represent a peculiar (generic or sub-generic) type ; Fandora 

 dilata is evidently a right valve of a ClidiopJiora, and F.primcevais a true Fandora, 

 Clidiophora also occurs fossil in North American tertiary deposits, but the genus 

 evidently had formerly a larger geographical distribution, as illustrated from its 

 occurrence in the Paris basin. 



No cretaceous forms are referable with sufficient certainty to Fandora or to its 

 allies, and still less certain are those from Jurassic and older deposits. When the 

 Valves of fossil species are found closed, particular attention is required not to 

 confound them with the shells of Toldia and other allied genera of the Nucuiibm. 



b. &ul-family, —TEE ACTING, 

 Shells very thin, with a more or less distinct nacreous layer inside ; form 

 tellinoid, being usually compressed, rounded in front, somewhat less high and sub- 

 truncate behind; animal with a small digitiform foot and moderately prolonged 

 siphons, separated the greater part of their length, but rmited at the base. 



6, Tellinomya, Hall, 1847, (Palseont. of New York, I, p. 151). Shell with- 

 out any hinge-teeth or crenulations, beaks entire ; form typical, rounded and higher 

 in front, lower and sub-truncate behind. 



This is apparently the oldest form as yet known of this group of shells ; it has 

 1)een proposed for a palaeozoic species from North- America, but there are in simi- 

 larly old rocks of other countries also species found which are referable to this genus. 

 The shell very probably had a thin external ligament, but the margin is not bent 

 in as in Thracia, nor is there, according to Hall, any other tooth present which 

 may have supported a cartilage. 



7. AsthenothcBrus, Carpt., 1864, (Ann. Mag, Nat. Hist,, XXII, 811). Shell 

 like ^ Thracia, hinge without teeth, cartilage situated under the umbones. The 

 species Ast. viUosior is a recent shell from Cape St. Lucas ; its external generic 



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