880 PELECYPODA. [Filibranchia. 



Diameter Ant. -post., 46 mm. ; dorso-ventral, 43 mm. : thickness, 

 20mm. 



Type in the Mus. Hist. Nat., Paris. 



Hob. North and South Islands of New Zealand : Bay of Islands ; 

 Hauraki Gulf, in 25 fathoms ; near Cuvier Island, in 38 fathoms ; 

 Cook Strait ; Dusky Sound. Stewart Island. 



Remarks Although provided with a byssus, this mollusc has been 

 observed swimming about. (Captain Bollons.) 



The species is not common at all, and good specimens are only 

 obtained by dredging. During the voyage of the " Astrolabe " only 

 two empty shells were collected. 



Fossil in the Miocene and Pliocene. 



Subgen. 3. PSEUDAMUSIUM, H. and A. Adams, 1858. 



Pseudamusium, H. and A. Adams (after Klein), Ad. G.R.M., ii, 1858, 553. 

 Type : P. hybridus, Gmel. 



Shell small, thin, more or less translucent ; the sculpture, if any, 

 feeble ; inner face of the disc without lira? ; disc with or without 

 Camptonectes striation, frequently with concentric imbrication. 



Sect. 1. CYCLOPECTEN, Verrill, 1897. 



Cydopecten, Verrill, Trans. Conn. Acad., x, 1897, 70. Type : C. pustulosus, 

 Verrill. 



Sculpture discrepant on the two valves, the right valve having the 

 concentric and the left valve the radial elements most pronounced ; 

 valves usually flattish or compressed ; whitish, thin, with delicate 

 sculpture. 



From shallow to deep water. 



KEY TO SPECIES. 



A. Left valve with 7-8 radiating lirse, crossed by concentric lamellae aviculoides. 



B. Left valve with numerous radial riblets, crossed by concentric 



threads, the points of intersection tuberculate . . transenna. 



1. Pecten aviculoides, E. A. Smith, 1885. Plate 52, figs. 2, a. 



Perten aviculoides, E. A. Smith, Chall. Rep., xiii, 1885, 305, pi. 22, f. 5, 5. 

 Cydopecten aviculoides, Smith : Hedley, Rec. A.M., v, pt. 2, 87. 



Shell small, very inequivalve, slightly oblique, thin, dirty-white. 

 The right valve is very slightly convex at the centre, and fits, as it 

 were, within the other valve, its very thin margin being upcurved 

 and appressed to the outer edge of the other valve. It is sciilptured 

 with a few lines of growth, and has under the microscope a minutely 

 shagreened appearance. The left valve is much deeper, and ornamented 

 with coarse concentric lamellae, which, crossing about 7 or 8 strongish 

 radiating lira?, produce a coarsely cancellated surface. The cars in 

 this valve have only the concentric lamellae, and are about equal in 

 size, the posterior being sinuated at the side. The anterior auricle in 



