[NVERTEBRA.TE PALEONTOLOGY. 45y 



ol the iirxt within, but the last one in the adult becoming proportionally less 

 deeply embracing; umbilicus very narrow, and rather deep in the young, 

 but proportionally wider in the adult; surface ornamented with numerous 



small, bifurcating, slightly flexuous costse, that arc larger near the umbilical 

 side, and on the last turn of medium and large sized specimens become pro- 

 portionally somewhat more prominent, more curved, and suddenly bifurcate 

 near the umbilicus, and again divide and subdivide into numerous smaller 

 ones, so that their number, including others intercalated between, amounts to 

 from five to seven times as many where they pass straightly over the periph- 

 ery, as near the umbilical side ; body-chamber forming at least the entire 

 outer volution. 



Septa crowded, and divided into five lobes and five sinuses on each side ;* 

 siphonal lobe longer than wide, truncato-suboval in form, with three prin- 

 cipal branches on each side, the two terminal of which are larger than either 

 of the other pairs, nearly parallel, and somewhat bifurcating and digitate, the 

 terminal subdivisions being long and slender, while of the lateral branches 

 the middle pair are larger, and more or less subdivided; first lateral sinus 

 about as wide as the siphonal lobe, but shorter on the umbilical side, and 

 very deeply divided into two unequal branches, of which that on the siphonal 

 side is larger, and also deeply bipartite, with its branchlets deeply subdivided 

 and obtusely digitate, while the other main branch is very slender, with 

 alternating, clavate and sinuous branchlets; first lateral lobe narrower and 

 shorter than the siphonal,! bipartite at the end, but with the two slender 

 bifid and digitate terminal branches unequal, the inner division of that on 

 the umbilical -side being longer than the corresponding one of the other, and 

 when crowded over more nearly on a line with the middle of the body, giv- 

 ing a decided tendency to a tripartite arrangement of the whole y second 

 lateral sinus scarcely more than half as long and wide as the first, and nearly 

 equally bipartite at the end, the two divisions being bifid, and, like the one or 

 two alternately-arranged lateral branches on each side, with sinuous margins ; 

 second lateral lobe about two-thirds as wide and long as the first, with 

 more or less tendency to a tripartite arrangement at the end, the divisions 

 being slender, digitate, and so arranged that one of the three might with 

 almost as much propriety be viewed as a lateral, alternating with a smaller 



* The fifth very small lobe is not represented in fig. 3c of plate 24 (see foregoing out). 

 t The difference in the lengths of these two lobes is not in all the septa so marked as in that 

 figured on plate 24. 



