30 



twisted on itself rather than regularly coiled. The upper half of the 

 volution bears two prominent, acute and distant, spiral keels, which 

 are separated from each other by a bi-oad concave groove. Above the 

 upper keel, which forms a distinct shoulder to the volution, the surface 

 is obliquely flattened or slightly concave, and below the second keel 

 the surface is somewhat convex. At the base of the earlier half of the 

 last volution there is a third keel, but on the later half of the same 

 volution this basal keel is continued as the outer margin or boundary 

 of the mouth on the columellar side, and ultimately becomes confluent 

 with the basal portion of the outer lip. The apertui-e appears to have 

 been nearly circular, and it is certainly broadly and effasely expanded 

 at its base. In addition to the keels the exterior of the test, which 

 seems to have been rather thick, is marked by veiy faint, fine and 

 flexuous, transverse striae of growth. 



Durham, Mr. J. Townsend, 1883 : a single specimen, with most of 

 the test preserved but with the apical portion and part of the outer lip 

 broken off, 



e 



It is at present doubtful whether this shell is a Murchisonia, allied to 

 but perfectly distinct from the M. helicteres of Salter, — a second species 

 of Oodonocheilus, or, as already suggested, a new generic type. Prom 

 ]\£. helicteres it differs not only in the number and arrangement of its 

 spiral keels, but also in the fact that although much drawn out in the 

 direction of its length, the spiral tube of which it is composed is so 

 tightly twisted on itself that its volutions are in contact throughout on 

 their inner faces and not entirely free and disconnected. 



The aperture of M. tropidophora, so far as known, seems very similar 

 to that of Codonocheilus, but in the only species of that genus yet 

 described all the volutions but the last are regularly spiral. 



The circumstance that the basal keel at the commencement of the 

 body-whorl in this species is continuous with the raised margin of the 

 mouth on the columellar side and that it finally becomes confluent with 

 the outer lip at the base of the shell, seems to the writer to be a unique 

 feature among gasteropoda and one which strongly favours the idea 

 that the present species may prove to be the type of a new genus. 



Genus Tryblidium, Lindstrom. 1880. 



Fragmenta Silwica (Stockholm), page 15. 



" Testa e stratis fibrilloso-prismaticis, osculis minutissimis perforatis 

 contexta, modice elevata, apice prope marginem anteriorem posito et 

 plerumque detrito, ita ut strata testes interna deteguntur. Impres- 

 siones musculares numerosse, per sex paria in orbiculo elongate ordinatee, 



