16 PKOCKEDINGS OF THE JIALACOLOGICAL SOCTRTV. 



costoe, and near the commencement of the second whorl the inner or 

 umbilical end of each costa becomes a little more swollen ; near the 

 base of the body-chamber this swelling diminislies. and before the 

 completion of the second whorl, i.e. on the body-chamber, the outer 

 nodes also disappear ; then the folds themselves gradually become 

 obsolete, the last half of the body-chamber being almost smooth. The 

 whole of the surface of the test is ornamented with very line regular 

 raised lines ; on the first half-whoi-1 these lines pass obliquely back- 

 ward across the lateral area and traverse the peripherj^ in a deep broad 

 backwardly - directed sinus ; when the whorl suddenly becomes 

 swollen the lines become more nearly direct, and on the rest of the 

 shell they are parallel to the costae over the lateral area and the 

 lateral ventral zone as far as the margin of the periphery, whilst 

 they cross the latter in a broad deep backwardly-directed sinus ; in 

 the adult the peripheral portion of the test bears also extremely 

 faint longitudinal lines. The ' normal line ' in the centre of the 

 peripheral area is displayed in an internal cast, more especially on the 

 body-chamber. Muscular attachment consisting of a finely punctated 

 semi-oval area on each side close to the base of the body-chamber, that 

 passes dorsally into a narrow punctated band which crosses the dorsal 

 surface of the body-chamber, forming the impressed zone on a broad 

 shallow sinus ; ventrally the two semi-oval areas are connected by 

 an exceedingly narrow, but apparently non-punctated band, crossing 

 the peripheral area close to the edge of the base of the bodj-- chamber. 

 Epidermids consisting of very fine puncturations, visible only with 

 a very strong lens, that exhibit a tendency to dispose themselves in 

 lines, which on the inner area (or umbilical zone) of the whorl pass 

 rather more obliquely backward than, but elsewhere have the same 

 direction as, the shell-ornaments. 



Tlie larger of the British Museum examples (T^o. C. 5277) has been 

 selected as the type-specimen, because it displays all the characters 

 of the species (PI. II, Figs. 1-4). It is 27 5 mm. in diameter and 

 consists of a little more than two whorls, the anterior part of the 

 body-chamber leaving the coil veiy slightly ; it has been broken 

 in such a manner as permits of the examination of the different 

 stages of growth of the shell. The first half-whorl, ornamented 

 with simple recliued raised lines, constitutes the nepionic stage ; 

 this passes with a rather abrupt swelling into the neanic stage with 

 its incipient costoe and distinct lateral ventral zone, and this in 

 turn passes almost imperceptibly into the ephebic stage with its 

 distinct nodose costae, the geroutic stage being indicated on the in- 

 ternal cast of the body-chamber by the weakening of the ornaments 

 and the gradual disappearance of the lateral ventral zone.' The 



1 In the Triassic example of Pleuronautihis superhis figured by Mojsisovics 

 (Abhandl. d. k.-k. geol. Eeichsanst. , Bd. vi, 1873, pi. iv, figs. \a-c) and 

 Hyatt (" Phylogeny of an acquired characteristic": Proc. Amer. Philos. Soc, 

 vol. xxxii, No. 143, 1894, pi. xii, fig. 3), the nepionic stage, consisting of an 

 entire whorl, is also striated and passes quite abruptly into the costated 

 neanic stage. 



