840 PELEC'YPODA. [Protobrarwhia. 



posterior end elongated, very gradually tapering, truncated at the 

 end, the dorsal margin straight ; basal margin lightly convex. Dorsal 

 areas not differentiated. Sculpture : Surface smooth, with fine con- 

 centric growth-lines ; 2 very inconspicuous ridges extend from the 

 umbo to the upper and lower angle of the posterior truncation of the 

 valves. Epidermis thin, polished. Colour yellowish-white. Interior 

 white, polished, finely radially striate ; margins smooth. Him/o- 

 piate very narrow and long, curved anteriorly, straight behind, with 

 two series of teeth ; the anterior teeth are V-shaped, and number 5 to 

 7 ; the posterior teeth are very long and narrow, imbricating, and 

 their number is usually 11. The two series of teeth are separated by 

 a narrow and very oblique resilifer, directed backwards. Adductor- 

 scars inconspicuous. Pallial line simple, with a tongue-shaped pos- 

 terior sinus. 



Length, 18 mm. ; height, 6-25 mm. (Pliocene, type). Length, 

 13-8 mm. ; height, 3-85 mm. ; diameter, 1-8 mm. (Recent shell). 



Type, from the Pliocene of Petane, in the Canterbury Museum, 

 Christchurch. 



Hob. Dusky Sound, in 5 fathoms (A. Hamilton) ; off Great 

 Barrier Island, in 110 fathoms. 



Remark. The fossil form is considerably broader in proportion 

 to length. The proportion of height to length is 1 : 2-9 for the fossil, 

 and 1 : 3-5 for the Recent shells. 



Fossil in the Pliocene. 



ORDER 2. FILIBRANCHIA. 



These are Pelecypoda whose main character is the possession of 

 gills formed of parallel, ventrally directed, and reflected filaments. 

 The successive filaments are joined together by cilia disposed in 

 " ciliated discs." The foot is generally provided with a highly deve- 

 loped byssogenous apparatus. The order comprises five suborders 

 Anomiacea, Arcacea, Mytilacea, Pectinacea, Dimyacea. 



SUBORDER 1. ANOMIACEA. 



Very asymmetrical animals, with a single large posterior adductor 

 muscle. The heart is not contained in the pericardium, lies dorsal 

 of the rectum, projects into the pallial cavity, and gives off a single 

 and anterior aorta. The reflected borders of the inner gill-plates of 

 either side are fused together in the middle line. The gonads open into 

 the kidneys, and the right gonad extends into the mantle. Dioecious, 

 marine. 



Shell thin, monomyarian, not alate ; edentulous or isodoiit, usually 

 sessile ; shell-substance nacreous, tubuliferous, with traces of a pris- 

 matic layer ; area obscure, usually small, amphidetic ; ligament 

 obscure, with an alivincular internal resilium. 



