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Pinna. Shell longitudinal, wedge-shaped, equivalve, gaping at the sum- 

 mit, pointed at the hase, the beaks straight; hinge lateral, without teeth. 

 Ligament marginal, linear, very long, almost internal. 



P. rudis, P. nobilis, P. pectinata, P. ingens, 



' flabellum, ' squamosa, ' saccata, ' vexillum, 



' seminuda, ' marginata, ' varicosa, ' nigrina, 



' angustina, ' muricata, ' dolabrata, 1 fossil species. 



MALLACEA.— Five Genera. 



Ligament marginal, sublinear, either interrupted by indentations or serial 

 teeth, or wholly simple. Shell subinequivalve, foliated. 



Crenatula. Shell subequivalve, flat, lamellated, irregular; no partic- 

 ular opening for the byssus; hinge lateral, linear, marginal, indented; inden- 

 tations serial, callous, hollowed into pits, which receive the ligament. 



C. avicularis, C. nigrina, C. viridis, C. phasianoptera. 



' raodiolaris, ' bicostalis, ' mytiloides, 



Perna. Shell subequivalve, flattened, rather deformed, texture lamellar; 

 hinge linear, marginal, composed of furrow-like, transverse, parallel, nonen- 

 tering teeth, between which the ligament is inserted. A posterior sinus, 

 slightly gaping, situated at the extremity of the hinge, for the passage of the 

 byssus; sides callous. 



P. ephippium, P, avicularis, P. marsiipiom, P. nucleus, 



' obliqua, ' femoralis, ' sulcata, 2 fossil species. 



' isognomon, ' canina, ' vulsella, 



Malleus. Shell subequivalve, rude, deformed, mostly elongated, sub- 

 lobate at the base; beaks small, diverging; hinge without teeth; an elongated 

 conical pit, situated under the beaks, traversing obliquely the facet of the lig- 

 ament. Ligament partly external, short, inserted in the sloping facet of each 

 valve. 



M. albus, M. normalis, M. anatinus, M. decurtatus. 



1 vulgaris, ' vulsellatus, 



Avicula. Shell inequivalve, fragile, rather smooth; base transverse, 

 straight; the extremities produced, the anterior caudiform or tail-like; a sinus 

 in the left valve; hinge linear, unidentate; a cardinal tooth in each valve, 



