48 fi. F. MATTHEW: ILLUSTRATIONS OF 



Snhsenws.—CAMEROTHECA, u. subgeu. 



-'a^ 



In describing these Hyolithoid shells of the t^t. John groiip, the simply eamerated 

 forms may be arranged lender the above name. 



General form that of a slender oval, or transversely oval, cone, with attenuated apex. 

 In the smaller part of the tube or cone there are several septa that divide off segments of 

 the tube from the body cavity (chamber of habitation). In most species this septate portion 

 of the cone is prolonged toward the apex into a narrow, attenuated tubule, formed during 

 the earliest stage of growth : the tubule is divided by transverse diaphragms (?) at regular 

 intervals and is more or less llexible. 



In these shells there is added to the ordinary body-cavity of the inseptate Hyolithes, 

 two spaces or regions, showing antecedent conditions of growth, differing from that of 

 the ordinary thecoid pteropod without septa. The first may be regarded as the larval region, 

 and is perhaps most instructively exhibited in a shell belonging to the genus Diphfheca, 

 described further on.' The tubule in this species is in the form of a flexible whip or tail, 

 and as preserved in the shale, is not nufrequently twisted from the plane, in which the 

 principal part of the shell is flattened, usually toward the ventral side, but sometimes to 

 the right or left. In some cases this larval region is preserved in the shale as a flattened 

 cylinder, in others as a very slender tubule, slowly expanding from the tip, but in all cases 

 it is crossed by distinct annulations, abont as far apart as the tube is wide, and of a firmer 

 texture than the interspaces. Whether the prominence of these nodes is due to the thick- 

 ening of the tube, or to the transverse diaphragms within, does not clearly appear, but the 

 latter alternative seems the more probable one. 



The mucroiiato point, in which the apex of most of the species of Hyolithes terminates, 

 may be seen to have been formed in the upper part of this tube in some specimens of 

 Di/ilotheca caudaia, but others do not show it. This mucronate point at the top of the tubule 

 in this species is the first thickening (or, in some cases, calcification ?) of the shell, and is 

 the known apex of most species of Hyolithes ; the absence of the tubule in later species is 

 perhaps to be explained by its being deciduous or becoming absorbed at an earlier stage 

 of growth. 



The possession of a septate region is perhaps the feature by which this subgenus of 

 pteropods can most easily be recognized, or at least separated from the commoner inseptate 

 forms of Hyolithes. This portion of the shell is marked by a more rapid expansion of the 

 cone, by added firmness' of texture in the shell, and in some cases by its more decided 

 thickening. The chambers, resulting from these septa near the apex of the shell, are not 

 known to be connected by a siphon. This part of the shell, as preserved in the shale, is 

 always more terete than the rest, and the walls of the cone above it, which enclose the body- 

 cavity, expand less rapidly than those of the septate portion. 



M. Joachim Barraude, in the third volume of his " Silurian System of Bohemia," men- 

 tions several species of Hyolithes that possessed chambers near the apex of the shell, 

 but he makes no reference to the attenuated tube such as is found at the apex of some of 

 the Acadian species. In the forms which he figures, the septa are closer together (in most 

 of them much closer) and more numerous than those of the theciform shells of the Acadian 



Biplothcca Hyattiuna, var. caudata. 



