MUSEUM OF COMPARATIYE ZOOLOGY. 207 



valves. Other characters seem equally interchangeable, such as the armature 

 of the Ups, which may be internally striate or smooth, externally smooth, papil- 

 lose, or arborescent. 



All these facts confirm me in my belief that the subdivisions of the group 

 may advantageously be limited to a comparatively small number. 



SCBGENUS JANIEA SCHUMACHEB. 



Pecten (Janira) hemicyclica Ravbnel. 



Janira hemicyclica Tuomey & Holmes, Miocene Fos. S. Car., p. 25, pL viii. figs. 



1-4, 1855. 

 Pecten hemicyclicus Ravenel, fide T. & H. 1. c. 



Plate VI. Tig. S. 



Two lower valves of this species were dredged on the west coast of Florida 

 by the Bache in 19 fathoms. It is found not very rarely on the east and west 

 coasts of South Florida, and often identified as P. ziczac. The ribs on the flat 

 valve difl'er greatly in diflferent specimens, being sometimes obsolete and some- 

 times very strong. The color of this valve is much as in P. ziczac. The color 

 of the convex valve and its sculpture are quite different from those of P. ziczac, 

 which grows to a considerably larger size at present. The fossil specimens 

 of hemicyclica, as figured, are larger than any recent ones I have yet heard of, 

 The very young of this species are externally indistinguishable from the fry of 

 P . magellanicus Gmelin and Amusium pleuronectes. The transverse rugosities 

 or grooves of the hinge-line referred to in Pseudamusium thalassinum are well 

 marked in the fry of this species, and very evident traces of them are visible 

 in the adult. In the young they occupy a lanceolate area on each side of the 

 cartilage pit, and are shown in the figure, considerably magnified, on Plate VI. 

 These shells and some other young fry are not to be distinguished from young 

 Pecten similis Laskey, of most collectors. I find fully half the " P. similis " of 

 the Jeffreys collection to be of this character. Many of them might have 

 grown to be that species, but many probably might not. Unusual localities, 

 such as Korea or Jamaica, quoted for P. similis (genuine) on the authority of 

 Dr. Jeffreys, should be suspected or held for more information. 



Subgenus AMUSIUM (Bolten) Schumachek. 



Historical Synonymy. 



Amusium Rumphius, Amboinische Rariteitkamer, pp. 144, 188, pi. xlv. figs. A, B, 

 1705. Klein, Tent. Meth. Ostrac, p. 134, 1753. Martini, Verzeichn. 

 Samml. Nat., 1774. 



Synonymy Proper. 



Amusium Bolten, Mus. Boltenianum, ed. i. p. 165, 1798 ; Pecten pleuronectes auct. (no 

 description or type mentioned). 



