TlIIRTY-i:iGHT EMlillYuNIC UNIONID.K. 45 



but I think, by the l)undle of muscular libro which, attached to the interior ol" the 

 valves, serves to close them together. The adductor muscle is placed towards the 

 dorsal margin, and consists of a single group, below the middle of the dorsal line. 

 It is evident that at this period, the members of this family are unimusculose, after- 

 wards becoming himmculose. In this respect, they have some resemblance to the 

 genus ifjiZ/ena (Fer.)=J.casto«, D'Orb.* 



There is no doubt but that the young of some species are attached by a filament, 

 which anchors them to the mother, or some permanent object, and this is probably 

 the case with those of the whole family. During this period of anchorage, the single 

 muscle which connects the two valves no doubt separates into two, removed, the 

 the one to become the anterior adductor, and the other the posterior adductor. 



It will be observed that the figures, on the plate, of all the species have a double 

 line along the margin. This is the case in all the young which I have examined in 

 this state. In some it is broader than in others. It may be occasioned by the at- 

 tachment of the palleal margin. 



The spur or hook, Avhich is mentioned on a previous page, is a very remarkable 

 appendage to the embrj-onic shell. Existing only in the species which are angular 

 at the base, with the exception of the two Uniones, represented as wedge-shape, and 

 which have two angles ; they bend inwards, and are flexible, and are not observable 

 when the valves are closed, unless faintly through the diaphanous valve. They dif- 

 fer in some species in size, and slightly in form, and the outer side is furnished with 

 two or more rows of apparently hard tubercles, extending from the base to the point. 

 In Margaritana rugosa, the hook is about one-tenth of a millimetre in length. In some 

 cases, these, when examined hoi'izontall}', will be observed to be serrate, as fig. 38 

 in the plate exhibits. In the two remarkable species represented as wedge-shape, 

 U. Icevissimm, (nobis,) and U. alatus, Say, at the two angles of the basal margin, 

 there is a hook on each valve. There are, therefore in these, four to each individual. 

 They are exceedingly minute, and could with difficulty be detected. No granu- 

 lations could be distinguished on the hooks of these two species. 



As regards the forms of the branchial uierus of different species, it is not my in- 

 tention here to describe them. That part of the economy of this family will be fully 

 entered into, when the soft part of the species are under consideration. Two saclcs, 

 taken from the branchial uterus of two species, are represented on the plate, fig. 16* 

 and fig. 38'. They are both of unusual form, and the young represented of these two 

 species were taken from these sacks. 



The first, from U. Woodward iaims, is clavate, large and blunt at the inferior 

 end and pointed at the superior one. It is represented here twice the natural length 



* Sec Rev. and Mag. de Zool. vol. iii.. i>. 1>*}. Tor genus Acoslini, IVOrli. and my oliservations on it, .Tournal 

 Acad. Xat. Sci. Nov. 1851. 



12 



