58 ZOOLOGY. 



Farn. 1. Mactridm. Hinge with an erect Y-shaped tooth, ligament 

 internal. The genus Mactra., Linn. {pi. 76, fig. 40), includes sub-trigonal 

 shells which are slightly gaping; hinge with a concave tooth to receive the 

 ligament; two additional striated teeth near the hinge. Tliis genus 

 contains shells which sometimes attain a considerable size. M. solidissima^ 

 (yhemnitz (Gould, Invertebrata of Massachusetts., p. 51), of the United 

 States coast, is sometimes found six inches long and four in height. In this 

 family the foot passes through an opening in the mantle, and the two 

 siphons, although distinct, are inclosed in a common integument. Some 

 authors place this family among the Elatobranchia, Lutraria lineata., Say, 

 American Conchology {pi. 9). L. pllcatella^ Lamarck, vol. vi. p. 93. 



Fain. 2. Myida'. This family is differently divided by different authors, 

 some including the two gr()Ui)S of which Osteodesraa and Solemya are the 

 ty})es, whilst others believe that these should form distinct families. In 

 Mya the shell is gaping, and there is a broad spoon-shaped tooth projecting 

 from the hinge to the opposite valve, where there is a cavity to receive it. 

 The labial palpi are long, stout, and pointed. The mantle is closed, except 

 a small anterior opening for the passage of a slender foot, and the ordinary 

 openings for the siphons, which are very long, and inclosed in a common 

 envelope. This genus buries itself in the sand, with the exception of the 

 end of the siphons. Mya aTenaria., which has a shell three or four inches 

 long, inhabits both sides of the Atlantic, between high and low water-mark. 

 It is extensively used in New England as food, under the name of clam., 

 a name which is applied to Yenus mercenaria in the middle States. The 

 genus Corbula forms part of the restricted family of Myada?. 



In the opinion of some authors, Osteodesma and the allied genera Lyonsia, 

 Periploma, Thracia, and Anatina, form a family, although the aggregate of 

 tliese genera seems to have no higher rank than that of a sub-family. 

 Some of the shells are of an extremely delicate texture ; these have no 

 cardinal teeth, and there is an ossiculum or accessory bone forming part 

 of the mechanism of the hinge, upon which Deshayes has founded the 

 family name Osteodesmacea. Mya norvegica was at first the type both of 

 Lyonsia., Turton, 1822, and Osteodesma., Deshayes, 1835, but the latter 

 author subsequently admitted Lyonsia, and applied his own term to a 

 different group. Couthouy's paper in the third volume of the Boston 

 Journal of Natural History, and Gould's Invertebrata of Massachusetts, may 

 be consulted upon these genera. 



Fam. 3. Solemyidm. The genus Solemya is by some considered as 

 forming a distinct family, whilst by others it is placed in the family of the 

 Solenidae. Some of its characters are very distinct ; the branchiae are thick, 

 and single upon each side, and they are divided so as to resemble those of 

 Crustacea. The foot is truncated and shaped like a sucking disk; the 

 })Osterior half of the mantle is closed, the siphons are short, and the 

 periostraca of the shell projects in a flap, far beyond the calcareous 

 j)ortion. 



Fam. 4. Saxicavidce. This family includes Saxicava and Byssomia. 

 The first contains small species and secretes a byssus. The cardinal teeth 

 262 



