56 PROCEEDINGS OF THE ACADEMY OP 



centrals, but uns3'mmetrical. Marginals quadrate, with one broad, 

 oblique, long, bluntly bifid inner cusp, and one or two very short 

 rounded side cusps. 



ThelidomiiH. We have examined H. aspera, Fer. (Am. Journ. 

 Conch., YI. 204); discolor, Fer. (ante, p. 51); notahilis, Shuttl. 

 (Amer. Journ. Conch., VII. 177); Zi?na, Fer. (ante, p. 51); and 

 provisoria, Pfr. (Ann. Lye. N. Y., x. 347). 



There are 8 decided stout ribs on the jaw of aspera, 7 on that 

 of discolor and lima, on provisoria we find 10-15 ribs, less decided 

 than in the other two species. 



I figure the lingual dentition of H. discolor (plate X., fig. 1). 

 H. aspera agrees with it. The marginal teeth of provisoria and 

 notahilis agree with those of aspera and discolor, but as the centrals 

 and laterals differ in the shape of the plates and the development 

 of the cusps, I also figure the central and lateral of nofabilis 

 (plate IX., fig. 10), with which prouz'soWa agrees. 



Cysticopsis we know by tumida, Pfr., and pemphigodes, Pfr. 

 For description and figure of the former see Ann, N. Y^. L3'C. 

 N. H., IX. 213, f. 3 ; Am. Journ. Conch., YI. 203, f. 1. Finding the 

 dentition o^ pemphigodes to be ditferent (Am. Journ. Conch., YII. 

 177) I here figure it (plate IX., fig. 1). 



Plagioptycha. We have published iT". ?oa;o(7on, Pfr., Albersiana, 

 Pfi'., monodonta, Lea, diaphayia, Lea, and macroglossa. (See Am. 

 Journ. Conch., YII. 177, 178.) The}' all agree in having a ribless 

 jaw with blunt median projection to cutting edge, and in dentition 

 such as I figure for H. macroglossa, Pfr. (plate X., fig. 10). 



Pohjmita We have elsewhere (Ann. N. Y. L3'C., x. 341) pointed 

 out the necessit}^ of revising this subgenus. The t3'pical species 

 muscarum, and H. pticta which must be removed to it from Lio- 

 chila, have a ribless jaw without median projection to the cutting 

 margin (see Am. Journ. Conch., YI. 204, pi. IX., f. 4, 10). The 

 lingual dentition of both species agrees (see ibid, and Ann. X. Y". 

 L3'C., X. pi. xvi. fig. 14). It is entirel3' diflerent from that of an3- 

 species now known. 



Hemilrochus must be used as the name for the balance of the 

 subgenus Polymila of von Martens. We have examined varians, 

 Mke. (Amer. Journ. Conch., YII. 206, and L. and Fr. w. Shells, I. 

 185) ; Troscheli, Pfr. (Ann. X. Y^. L3-c., x. 343); graminicola. Ad. 

 (Am. Journ. Conch., YII. 178) ; gallop)avonis, Yal. 343. (N. Y. Am. 

 X. 343.) All agree in having an arched ribless jaw with blunt 



