PELECYPODA. 829 



with interlocking teeth, small pits or depressions in each valve corre- 

 sponding to the teeth in the other. The teeth are distinguished as 

 cardinal, or those immediately below the umbo, and lateral, or those 

 to either side of the cardinals, the latter being also distinguished 

 as anterior and posterior laterals, according as they are before or behind 

 the umbo. 



Some bivalves (Anodonta, Ostrea, many Mytilus) have no hinge- 

 teeth at all ; in others the laterals are wanting (Psammobia, Diplo- 

 donta). In the Arcadce the hinge consists of a number of very similar 

 denticles, which are often serrated like the teeth of a comb. 



ORDER 1. PROTOBRANCHIA. 



These are Pelecypoda whose distinctive character is the possession 

 of gills with flat and non-reflected filaments disposed in two rows 

 on opposite sides of the branchial axis. The mantle is provided 

 with a hypobranchial gland lying on the outer side of each gill. The 

 foot has a plantar ventral surface, and the byssogenous apparatus 

 is but slightly developed. The nervous system generally presents 

 a distinct pair of pleural ganglia, and the otocysts are generally open. 

 The gut may be provided with a relic of the pharyngeal cavity, which 

 in some cases is furnished with 2 lateral glandular sacs. The auricles 

 of the heart are muscular ; the kidneys are rather simple in structure, 

 and glandular throughout their extent. The sexes are separate : the 

 gonads have retained their primitive communications with the initial 

 or internal extremities of the kidneys, but. as the two branches of each 

 kidney have acquired a secondary communication at their anterior 

 ends, the genital products pass direct to the external orifice of the 

 kidney by this passage. 







Fam. SOLEMY1DJE, Gray. 

 Solenomyidve. 



Animal : In the gills one row of branchial filaments is directed 

 dorsally and the other ventrally. The mantle-lobes are united vent- 

 rally, attached in front to the epidermis and valves by a broad sur- 

 face, leaving no distinct pallial line ; there is a single posterior siphonal 

 and anterior pedal foramen in the mantle. The labial palps of each 

 side are fused together. Adductor subequal. Animal dioecious, 

 marine, burrowing. 



Shell soleniform, equivalve, low-beaked, edentulous, gaping, with 

 the anterior end longer and the epidermis conspicuous, exceeding 

 the valves ; area obscure or none ; ligament amphidetic, parivin- 

 cular, becoming internally posteriorly : posterior muscle-scar with 

 a thickened ray in front. 



Silurian to Recent. 



