210 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 54. 



periostracum extremely thin and usually absent except in the most 

 protected places ; the color tending in many cases to a purplish tint. 

 In Colits, on the other hand, the periostracum is generally conspicu- 

 ous, strongly adherent, usually smooth or even polished, though 

 occasionally villous; the outer layer of the shell beneath it of an 

 opaque chalky consistency and generally whitish. When the villosity 

 of the periostracum is worn the basis may remain smooth and even 

 acquire a polish. It is usually of a yellowish or greenish brown. 

 The operculum is usually rounded-triangular with apical nucleus, 

 the inner side with a thickened margin of a vitreous appearance. 

 This, however, may be reduced to a thin, hardly perceptible varnish. 

 The operculum may be shortened and the apex curved to the left, a 

 tendency which in Mohnia is increased until the operculum as- 

 sumes a subspiral form. In Ancistrolepis it becomes fan shaped, 

 solid, with the apical part much prolonged beyond the attachment to 

 the opercular gland, reminding one of the spurlike end of the oper- 

 culum in Stromhus. In Beringius it becomes short and rounded- 

 quadrate, the nucleus at one side. 



Wliichever set of characters are selected to divide the genera into 

 groups, it will be found that the other characters, each in its own 

 group, will provide a parallel set of forms. Thus it becomes ex- 

 tremely puzzling to decide which characters shall carry most weight, 

 and whatever decision is arrived at there will be a reasonable oppor- 

 tunity for differences of opinion among systematists. 



The dentition among the species examined seems to show compara- 

 tively little variation, chiefly in the presence or absence of minor 

 cusps. 



It is somewhat surprising that some authors, even at this late day, 

 will accept the prelinnean and frankly polynomial names in the 

 work of Klein and oppose the adoption of the binomial and prop- 

 erly proposed names of the Museum Boltenianum. The probable 

 explanation is that the latter until recently has been difficult of access 

 and Klein's miserable Tentamen is comparatively common. At all 

 events Klein's polynomials have fortunately no standing in zoological 

 nomenclature. Morch in 1852 adopted the name Sipho for Murex 

 islandicm Gmelin, but it had previously been used by Fabricius and 

 others and was not available. Moreover, the same species had been 

 adopted by Beck as an example of his genus Tritonofusus. In my 

 discussion of the history of the generic name Fusus in 1906, above 

 cited, I showed that by adopting the name Colus Bolten, for the group 

 typified by M. islandicus, the name Fasciolaria of Lamarck could be 

 conserved. Colus being prior to Beck's genus, the name Tritonofusus, 

 accepted by me in revising the family in 1902, must be relegated to 

 synonymy. 



