106 CEETACEOUS PELECTPODA 



ScL The species, like FapL Tauresii and others, called Ceronia by Gray, com- 

 bine with an elongately oval form the crenulations of the lateral teeth, and may 

 on this account be regarded as a sub-genus or section, but not as a distinct genus. 



4. Mactropsis, Conrad, 186 ? ; (see Am. Journ. Conch., 1865, I, p. 3). In the 

 Check list of eocene fossils (1866, p. 7,) Conrad places the genus in this family. 

 Judging from the figure of one of the species, Mactra Grayii, Lea, (Cont. to GeoL, 

 pi. I, fig. 10 = M, csquorea, Conr.), the shell resembles in form Anapa; it has in 

 the left valve either one bifid or two diverging cardinal teeth, a distinct anterior, 

 but less prominent posterior lateral tooth; it is said to possess a small obtusely 

 angular pallial sinus ; in external appearance it looks very much like a Crassatella. 



In the present seas about 45 species of Faphiibje have been observed. 

 Eossil species are always great rarities ; only a small number is known from the 

 tertiaries, but none from cretaceous, or older deposits ; at least none of the older 

 so called Mesodesmce have as yet been satisfactorily proved to belong to Faphia, 

 It is, therefore, not necessary to occupy ourselves for the present with this family 

 any further. 



X. Family,— SCBOBICULARIIB^. 



In this family the animals usually have a prolonged strongly compressed and 

 pointed, but not a high foot ; the palpi are very large, sub-triangular, striated, the 

 gills, of which two exist on each side, being smaller than the palpi ; the two 

 siphons are very long, unequal, thin, at their terminations mostly entire, the mantle 

 is for a very short distance united at the posterior ventral side, open, as usually, in 

 front and behind. 



The shells are sub-triangular, or more or less roundish, usually thin, equivalve, 

 slightly gaping posteriorly ; external ligament thin, but generally distinct ; hinge 

 with a cartilage-pit, situated either almost parallel to the areal margin, or in a pro- 

 jecting process; generally one or two small cardinal teeth in front of the pit; 

 lateral teeth present or obsolete ; pallial sinus large. 



When Deshayes first characterized this family (Traite elem. de Conch., 

 p. 317,) under Lattreille's name AMPniDEmiDJE, he placed in it four genera, 

 Cumingia, Semele (=AmpUdesma), Syndosmya (=AbraJ,Sind Scrobicularia C=Lavig- 

 non or Trigonella) ; to these Theora, Flectodon, Leptomya, Thyella, Montrouziera, 

 the fossil genus Falceomya, and several sub-genera now have to be added. They 

 may be divided into two groups, cuminqiinj^ and sjemelin^. The first possess a 

 solid shell with a special, more or less straight, or vertical cartilage process, and 

 are transversally elongated, much resembling some forms of the lutrariin^ ; 

 to this division Giimingia, Thyella, and Montrouziera belong. The others have a 

 thinner shell and a simple, posteriorly prolonged oblique cartilage-pit or groove; to 

 this group the remaining genera with a few sub-genera are referable; they form 

 a good passage to the TuLLmiDm. 



