1905.] NATURAL SCIENCES OF PHILADELPHIA. 347 



to as transverse ribs, or simply as ribs. When, in later stages, finer 

 transverse markings appear between the ribs, they are described as 

 riblets. Here the term is used principally for convenience, the actual 

 difference between ribs and riblets being one of degree. 



The angle which appears early on the rounded whorl is termed the 

 shoulder angle. On the ribs, especially at the shoulder angle, may 

 occur nodes or tubercles. These, in the early whorls, are mere rounded 

 knobs; but in the later whorls they become larger, higher and sharper, 

 and may therefore be referred to as spines. The writer considers that 

 the difference between the small rounded tubercle of the early whorls 

 and the large, sharp spine of the later whorl is one merely of degree. 

 The word spine is therefore used, throughout this paper, simply in a 

 descriptive sense, and implies no difference from the early tubercle, 

 except that it is larger and sharper. Both are produced by the same 

 fold of the mantle and every gradation between the two is observable. 



As stated in the Introduction, Volutilithes has been chosen as an 

 illustration on account of the normal and even course of its ontogeny. 

 The stages are often very distinct, and frequently are all represented 

 down to the senile condition in the later whorls. In addition, Voluti- 

 lithes petrosus is of unusual interest on account of its individual varia- 

 tion and its division into more or less definite races, which exhibit 

 different stages in the specialization of characters. 



In the early stages of Volutilithes petrosus there is a varying number 

 of smooth rounded whorls; but as a rule there are three or four. 

 The first ornamental feature to appear on the rounded whorl is the 

 transverse rib, which at first is simple (without tubercles) and usually 

 decidedly curved. Soon the simple ribs are followed by uncurved 

 ribs, bearing as a rule two tubercles, but sometimes more than two. 

 One of these tubercles is situated at the suture, the other at the position 

 of the shoulder angle. If more tubercles are present they are below 

 the shoulder angle tubercle. Sometimes the suture tubercle and the 

 shoulder angle tubercle are of the same size; but often they are of 

 unequal size, and then it is usually the suture tubercle which is at first 

 the larger. The suture tubercle, however, soon degenerates and be- 

 comes obsolete, while the shoulder angle tubercle increases in import- 

 ance and by its increase in size and sharpness makes the spine of 

 the later whorls. 



Coincident with or shortly after the appearance of the tubercles on 



the ribs, a more or less well marked shoulder angle develops. As the 



shoulder angle tubercles become larger, sharper and more prominent 



the transverse ribs degenerate. This process begins first above the 



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