52 G. F. MATTHEW: ILLUSTEATIOXS OF 



BIPLOTHECA, u. sen. 



TJuder this name the second genus of the Hyalithoid pteropods of the St. John group 

 may be described. 



Slender oval cones, somewhat triangular in section, with abbreviated or attenuated 

 apices. In the narrower part of the tube or cone, there arc several septa that divide off 

 segments of the tube from the body cavity (chamber of habitation). The body cavity is 

 separated from the ventral side by a thin partition supported by delicate transverse septa or 

 diaphragms. The apex in one species is prolonged into a narrow attenuated flexible 

 tubule with transverse annulations (diaphragms ?) at regular intervals. 



This genus differs from the last in the more rapid enlargement of the shell during 

 growth, and in having a firmer and (as preserved in the shales) rounder ventral side ; the 

 projecting ventral side is supposed to have been sustained by the lateral skeleton described 

 above. The skeletal feature is most distinct in the thick-shelled varieties of two species 

 which are found in the sandstones of l.b. In these it has been observed on both edges of 

 the ventral side, but has not been traced all the way around that side. Other varieties of 

 these species from the fine shales of the overlying measures do not exhibit this feature 

 with such distinctness. 



The species of this genus thus far met with in the St. John group are peculiar in hav- 

 ing the margin of the operculum angulated or notched at the meeting of the ventral and 

 lateral slopes. 



We find the following phases of growth in Hyolithes and its allies : — 



Primary annulated tube, probably divided byi „. , , , ,, 



, . , >■ Diplotlieca and Camerolheca. 



aiaphragnis > 



Simple conical tube Hiiolithc.t in part. 



Conical tube, septate near the apex HyoUlhcs in part, also Camcrnlliica and Diplotlicra. 



Conical tube with phragmato chamber at the) 



., <-4, 1 , •. \ Diplotluca. 



side 01 tlie riody cavity J 



The different phases of growth through which these pteropods passed suggest some 

 interesting comparisons with the Cephalopods. Professor Alpheus Hyatt, in his article on 

 the fossil Cephalopods, read at the Minneapolis Meeting of the American Association, 

 1883, concludes from his study of this group of animals that " the prototype of the Mol- 

 lusca must have had a globose embryo attached to the apex, the apex composed of a liv- 

 ing chamber opening into the protoconch or globose shell of the embryo without septa, 

 though possibly divided more or less by diaphragms. Diaphragms precede the forma- 

 tion of the septa in the embryo Ammonoid," and " the prototype of the subclasses, Tetra- 

 branchiata and Dibranchiata, must have been a form of the same type, with, however, a 

 single septiim or series of septa having closed cœca in place of a siphon." In these 

 words there is almost an exact description of the larval and septate regions of the shells of 

 Camerotheca and Diplotlieca ; and again there is a curious parallelism between the last 

 phase of Diplotheca and the cephalopodous genera, Phragmoceras, Ascocerns and Glossocems. 



DiPLOTHECA Hyattiana, n. sp. (Plate VI. Figs. 4 and 4^.) 



Form that of a somewhat elongated cone, flattened on the dorsal side, and near the 

 apex curved toward the ventral side ; the arched part is divided })y three or more trans- 



