INVERTEBKATES. 373 



notched at the base of the columella; surface smooth, or only 

 with obscure lines of growth. 



The shells belonging to this group have generally been supposed to be related 

 to Achatina and Bulimus. They have not, however, as remarked by Col. Port- 

 lock, the obtuse mammillated apex of Achitina {Polyphemus), to which they 

 have been sometimes referred, while their smooth, or even polished surface — 

 only showing obscure lines of growth under a magnifier — gives them a differ- 

 ent aspect from any of the land shells with which they have been compared. 

 In addition to this, the fact that all their associates, both in this country and 

 in Europe, are clearly marine types, is a sufficient evidence that they are not 

 related to any of these land shells. On the contrary, they seem rather to be 

 related to the Ulimidse, or some of the allied families. 



The Silurian group Subulites, Conrad, 1847, presents, so far as known, ap- 

 parently the essential characters of this genus, and if generically distinct, 

 evidently belongs to the same family. The close similarity of these types is 

 not so apparent in the particular species figured by us, as in P. elongata, of 

 Portlock (Geol. Loud., pi. 31, fig. 2), and P. fusiformis= (Polyphemus fusifor- 

 mis, Sowerby; Gcol. Trans., vol. v, pi. xxix, fig. 26), both of which are typi- 

 cal Carboniferous species of this genus. 



Where the aperture and columella cannot be seen, it is sometimes difficult to 

 distinguish these shells from elongated forms of Macrocheilus, from which they 

 differ in wanting the callosity and fold, or revolving ridge, of the columella, 

 characterizing that type, and in the truncated base of the columella. The lat- 

 ter character, and their smooth surface — destitute of the distinct curved striae 

 or costae, — distinguishes these shells from Phillips's Loxonema. 



The little shells upon which Prof. Hall proposed to found a genus Bulimella, 

 agree exactly in all their generic characters with Polyphemopsis. It is true 

 they are much smaller than the types of that genus, such as P. elongata and 

 P. fusiformis, but if we were to make size a generic character in this instance, 

 we would have, on the same principle, to create new genera for nearly all the 

 shells found at the same locality, since they are almost all diminutive in size, 

 although presenting exactly the generic characters of Murchisonia, Pleurotoma- 

 ria, Loxonema, Gonularia, Conocardium, and various other genera. The species 

 described by Prof. Hall under the preoccupied name Bulimella, are Polyphe- 

 mopsis oulimiformis, P. canalicidata and P. elongata. 



The presence of an inner lip below the middle of the columella, of some 

 species of this genus, is stated with doubt in our diagnosis, because we have 

 only seen it in one species (our P. peracuta), which may possibly not belong 

 properly to this genus. 



Unless this group includes Subulites, and some doubtful Devonian forms, it 

 would seem to be confined to the Carboniferous rocks. 



