THE DEVELOPMENT OF SCAPHITES 647 



At a diameter of 5 mra the shell is seen for the first time to be 

 sculptured. A slight wrinkling of the surface is first noticed, and 

 this soon becomes well defined costae, and a little later nodes develop 

 on the ventral shoulders. The costae are seen to begin on the sides 

 of the coil, running away from the nodes in one direction to the umbili- 

 cus, in the other across the venter. The sutures are in number of 

 lobes, and in every way, save in minor, secondary digitations between 

 the lobes, essentially adult. The internal lobes and saddles have 

 changed very much from the types seen in the embryonic stage. 

 These changes are shown in Fig. 3. 



The way in which the ribs and nodes begin is interesting, and may 

 be briefly outlined. First, there appears a slight wrinkling along the 

 sides of the whorl; then there is to be seen a swelling along the 

 ventral shoulder line. From these slight protuberances the costae 

 bifurcate in crossing the venter, but come together at the node on the 

 opposite side. Between these, and later, a short rib develops on the 

 venter, passing round on the sides afterward. In the adult stage 

 more ribs are intercalated in the same manner. On the abnormal 

 body chamber the ribs grow finer, and the nodes or tubercles cease 

 abruptly (Numbers 2 and 3, Fig. 1). S. nodosus quadrangular is is 

 the only one in all the forms studied to show nodes on the umbilical 

 shoulders. They are never very prominent, however. 



The abnormal body chamber or living-chamber, anormaler Wohn- 

 hammer of the Germans, marked not only by lack of sutures and 

 diminuation of sculpturing, but in another characteristic, that of 

 leaving the true spiral, is the last living-chamber. In the several 

 species and varieties this is by no means constant. Here are some 

 measurements, diameters at which this chamber begins: 



mm 



S. nodosus var. brevis - - - - 29 

 S. nodosus var. 30 



S. nodosus var. - - - - ""35 

 S. nodosus var. quadrangular is - - 37 



S. nodosus Owen has a very long body chamber, and, having 

 only one specimen, the writer could not break this off. It begins, 

 though, at about the same diameter as the other varieties. Scaphites 

 nodosus var. plenus differs in the adult from brevis, as will be readily 

 seen in an examination of Numbers 1 and 2, Fig. 1, and Numbers 5 



