A New Genus of Cambrian Pteropods. 151 



of the body-cavity is more ventricose, and the lower chambers 

 formed by the septa at the ap.:x are broken away, or from other 

 causes are wanting. Phragmotheca of Barrande comes nearer 

 the St. John genus in time, being of Upper Silurian age, but 

 as it is classed by some authors as a sub-genus of Pterothe6a 

 it is evidently widely different from the slender cones of the genus 

 Camerotheca* 



In the three phases of growth through which these pteropods 

 passed some interesting affinities are suggested. Professor Hyatt in 

 his article on the Fossil Cephalopoda read at the Minneapolis meet- 

 ing of the American Association, 1883, concludes from his study of 

 this group of animals that the prototype of the mollusca must 

 have had a u globose embryo attached to the apex, the apex com- 

 posed of a living-chamber opening into the protoconch, or globose 

 shell of the embryo, without septa, though possibly divided more 

 or less by diaphragms. Diaphragms precede the formation of 

 the septa in the embryo Ammonoid. This confirms Yon Jhermig's 

 opinion that Tentaculites was the prototype of the Cephalopoda, 

 since it has similar embryo and diaphragms. The prototype 

 of the sub-classes Tetrabranchiata and Dibranchiata must have 

 been a form of the same type, with, however, only a single septum 

 or series of septa, having closed coeca in place of a siphon," 



In these words we have almost an exact description of the larval 

 and septate regions of the shell of Camerotheca, a genus which, 

 originating earlier than Tentaculites, is more likely to have been 

 connected with the ancestors of the Orthoceratidae. 



Finally, I may remark that the form of the apex of the Castero- 

 pods of the St John group belonging to Division 1 (the portion 

 of the formation containing the Acadian fauna) is a straight tu- 

 bule similar to those we have described, though much shorter in 

 proportion to their size, and apparently devoid of diaphragms. 



* At the time of writing the above description of Camerotheca, I 

 had access only to the generic description of Hyolithes, in which no 

 reference is made to the occurrence of septa in the tube. Since then, 

 however, I have seen the specific description of Eichwald's typical 

 species H. acuius, from which it appears that this species had one of 

 the important characters of Camerotheca, viz., septa near the apex 

 of the tube. It will therefore be necessary to place Camerotheca as a 

 sub-genus of Hyolithes, characterised by its elongated form and 

 attenuated and flexible apex. 



