330 



MISS MCDONALD AND DE. TETJEMAN ON [vol. Ixxvii, 



Zygopleuba subrugosa, sp. nov. (Text-fig. 18.) 

 Dimensions of holotype. 



Length . 

 16 mm. 



JBreadtli. 

 33 per cent. 



Length of spire. 

 69 per cent. 



Spiral 



Stitural 



angle. 



angle. 



18° 



107° 



Holotype : Collected by Mr. J. Gr. Hamling, P.Gr.S., now in the British 



MiTseiim (Natural History) collection, G. 24882. 



A coarselv-eostate slender form, with wide concave sutures 

 separating the later whorls. 



Development. — The apical whorls are not preserved in the 

 type-specimen, but the earliest whorls present are convex and 

 ornamented with simple curved ribs, closely resembling Z, cf. 

 hlainvillei ; on the fifth whorl present the ribs have somewhat 

 coarsened, and this feature is more apparent on the remaining 



Fig. 18. — Zygopleura subrugosa, sp. nov. Soloty^oe. 



[a = Complete specimen, X 3. & = A portion enlarged to show the moulding 

 of the whorl and the shallow suture.] 



Avborls, where they become more widely separated, with a tendency' 

 to produce elongated tubercles on the anterior margin of each 

 whorl ; in one or two cases, two of the ribs appear to unite on the 

 anterior part of the Avhorl. In the early whorls the sutures are 

 normal and fairly deep ; but, after three or foui* whorls, the sutural 

 depression becomes wide and shallow, owing to the posterior portion 

 of each whorl being moulded over the anterior portion of that 

 preceding it (fig. 18 Z*). 



This species has apparently been evolved from some species of 

 Zygopleura comparable with Z. cf. hlainvillei ; but it differs from 

 that form in the coarser ornament and imbricating whorls, in which 

 characters it resembles Z. suh nodosa (A. d'Orbigny) and Z. verru- 

 cosa (Terquem). These two last-named forms, however, are 

 found at a much lower horizon {angulatus zone) : it is alsa 



