426 CRETACEOUS PELECYPODA 



Sow. Forms belonging to this sub-genus only occur in the mesozoic strata. The 

 two valves often are unequally strongly striated. 



3 e. Pseiulamusium, Klein, 1763. Shell usually fan-shaped, thin, sub-equi- 

 valve, valves smooth or finely radiately striated and plicated; ears of moderate 

 size, sub-equal or unequal, with straight hinge-line; byssal sinus of anterior ear 

 small; tyj)e, Fecteii exoticiis, Chem. f Fseudamusmm-Fecteii Icevis, &c., of Klein). 

 This should be retained only for species of the above type, like P. corneiis. Sow., 

 hyalinus, Poll, tigrlnus, Mull., nutans, Phil., &c., which are closely allied to 

 Amusium, differing from it by having the valves perfectly closed. 



3/. Syncijclonema has been proposed by Meek (1864, Smith. Misc. Coll., 

 No. 177, p. 31), for a small species formerly described as Pecten rigkla, Hall and 

 Meek f= J. IlalUi, Gabb, Am. Phil. Soc, VIII, p. 214). The valves are nearly 

 equal, ovately sub-orbicular, rather compressed, closed, smooth, or concentrically 

 striated, and with unequal ears. There appears to be no essential difference between 

 that species and Pseudamuskim as restricted, except that the shell is somewhat 

 higher than long or wide, while the species of Pseudamusium usually are wider 

 than high. A great number of cretaceous species differ in that respect from 

 Pseudamusium, and the name can, therefore, be appropriately reserved for them. 



4. Amusmm, Klein, 1753, fPleuronectia, Swains., 1840). Shell sub- 

 orbicular, subequivalve, rather thin and gaping in front and behind ; surface of 

 valves smooth or indistinctly radiately striated ; ears of proportionate size, sub- 

 equal, with a slightly angular hinge-line, the upper edges of the ears a little 

 ascending, the anterior ear of the right valve often with a very small byssal sinus ; 

 hinge-line generally with one or two radiating ribs from the centre, superseded by 

 the linear ligament ; cartilage pit of moderate size, triangular, central. Type, Am. 

 -pleuronectes, Linn. 



The two valves of Amusium are often differently coloured, and occasionally 

 one of them shows fine radiating ribs, while the other is perfectly smooth. This 

 character appears to have been rarely noticed in fossil shells, and therefore the 

 two valves are occasionally found described vmder two different names. Species 

 of the genus are not numerous in the present seas ; there are a few recorded from 

 tertiary strata, and a larger number of similar forms is known from mesozoic beds ; 

 but it is often difiicult to determine whether they belong to this genus. Many 

 species appear to have the valves closed, and these must, therefore, be referred 

 to the sub-genus Pseudamusium or Syncyclonema. 



5. Vola, Klein, 1763, (Janira, Schum., 1817, Neithea, Drouet, 1824). 

 Shell suborbicular, inequivalve, closed, radiately ribbed, right valve convex, left 

 flattened or concave, and often somewhat smaller ; ears rather large, sub-equal, the 

 anterior of the right valve generally with a conspicvious byssal sinus ; hinge-line 

 straight or nearly so, often slightly thickened at the margin, and with a few 

 ribs radiating from the centre ; cartilage pit triangular, central, deep. Type, Vola 

 Jacohcea, Linn., from European seas. The species which Klein figures as 

 " Indica,'" &c., is probably V. Sinensis, Sow. 



