INVERTEBRATE PALAEONTOLOGY. 431 



luil often becoming nearly or quite obsolete toward (lie aperture on the non- 

 septate deflected part of the outer volution — all occupied by the little nodes 

 of the lateral surfaces, of which about six to eight concentric rows may 

 usually be counted on each side of the volutions ; nodes of outer row around 

 each margin of the flattened periphery, larger than the others, and sometimes 

 compressed. 



Septa rather deeply divided into four principal lobes and as many sinuses 

 on each side of the siphonal lobe, which is nearly oblong in form, about twice 

 as long as wide, and bears three slender digitate main branches on each side, 

 the two terminal of which are a little longer than the others; first lateral 

 sinus as long as the siphonal, and a little wider — provided with three nearly 

 equal, slender, deeply incised and digitate terminal branches and smaller 

 lateral divisions ; first lateral lobe as long as the siphonal, and nearly of the 

 same breadth at its free end, where it is provided with two unequal branches, 

 the larger of which (that on the inner side) is subdivided into three digitate 

 branchlets, and the smaller into two, while its slender body supports one or 

 two small, partly digitate, diverging lateral branchlets; second lateral sinus 

 shorter than the first, and scarcely more than half as wide, with two nearly 

 equal bifid and serrated terminal branches, and several short, obtuse, 

 irregularly- notched, alternating lateral divisions, the sinuses between which 

 are so deep as to give the body a very narrow, zigzag appearance; second 

 lateral lobe a little more than half as long and wide as the first, and provided 

 with two bifid and digitate terminal branches, and one small, nearly or quite 

 simple, diverging lateral branchlet on each side of its slender body; third 

 lateral sinus shorter than the second, but of nearly the same breadth, with 

 a very slender body and two nearly equal, irregularly trifid subdivisions ; 

 third lateral lobe rather more than half as long and wide as the second, 

 and very similarly formed ; fourth lateral sinus half as long and wide as the 

 third, with two small, irregularly serrated, terminal branches ; fourth lateral 

 lobe small, and bifid at the end, the two divisions being very short and hi- or 

 tri-dentate. 



Length of largest example, 6.30 inches ; height of same, 5.70 inches ; 

 convexity, about 2.70 inches. 



The foregoing description of the septa was prepared from the specimen 

 represented by figs. 2, a, and 2, b, of plate 36, regarded as belonging to the 

 typical form of the species, as it seems to agree in all essential characters 



