166 FAMILY LAGENIDA. 



more than 100 fathoms. The marine Tertiaries of other regions are scarcely less rich in 

 Nodosarine forms ; so that we may consider this type to have an equally wide diffusion in 

 time and in space. 



Genus III. — Orthocerina (Plate XII, fig. 7). 



256. Hisfori/. — This is one of the numerous genera created hy D'Orbigny in 1825 for 

 the reception of the various modifications of the Nodosarine type ; and it is the only one 

 which we do not at present see adequate reason for reuniting with the rest, its combination 

 of characters being so peculiar as to differentiate it from every form of that series, as well as 

 from the other genera of the family Lac/enida. 



257. External Characters and Internal Structure. — The shell of this genus is formed of 

 a succession of segments arranged in a straight line, without any intermediate constrictions ; 

 these segments usually increase pretty regularly in size, and are either square or triangular 

 in their cross section, so that the whole shell has the form of an inverted pyramid, either four- 

 sided or three-sided, the faces being generally concave (Plate XII, fig. 7). The septal planes 

 are sometimes nearly flat, but are more often very convex. The aperture, whicii is a simple 

 circular perforation without radiating fissures or equal and regular denticulations, is somewhat 

 pouting, but does not generally project as a distinct tube. The angles sometimes become 

 sharply carinated by exogenous deposit; and thick, rounded, longitudinal costse may exist also 

 midway between the angles, as in the form designated Hhahdoyonium anomalum by Reuss (xc b), 

 these, however, being formed rather by the sudden convexity of the walls of the chambers 

 midway between the carinated angles, than by any great excess of exogenous deposit. 



258. Affinities. — This genus is obviously a member of the family Lagenida, to the 

 composite forms of which it is allied in its general mode of growth, the character of its 

 aperture, and the disposition of exogenous deposits on its surface. Its nearest relationship 

 seems to be to certain feebly-developed forms of TJingerina ; but in the rectilineal disposition of 

 its segments it conforms to the Nodosarian type. 



259. Geographical and Geological Distribution.- — ^The only recent example of this type at 

 present known is that mentioned by D'Orbigny (lxxiii) as having been obtained from the 

 West Indian seas. So far as we yet know, the earliest appearance of Orthocerina is in the 

 Oxford Clay ; it is afterwards met with in the Chalk formation and in the Paris Tertiaries. 



Genus IV. — Pqlymorphina (WiUiamson, Figs. 145 — 1.57). 



260. Historg. — The earliest notice of the shells of this very common but extremely 

 variable type, occurs in the work of Boys and Walker (ix) ; in which the generic name 



