﻿THE ACANTHODES-STOCK. 



331 



difference in the coiling of the umbilicus is a very marked feature, rendered more 

 apparent by the thinner form of substriata. It may be illustrated thus : 



substriata. simplex. 



Diameter . . 156 mm. 190 mm. ... 156 mm. 190 mm. 



Umbilicus . 52 „ 68 ... 49 „ 63 „ 



Umbilicus, one whorl back 21 ,, 27 ... 16 „ 22 



Umbilicus, two whorls back 10 ,, 12*5 „ ... 7 „ 9 ,, 



Sonn. substriata occurs at Bradford Abbas, and is represented by several 

 specimens in my collection. There are also some examples which are intermediate 

 between it and subcostata, and of these a young one is depicted in figs. 1, 2 of 

 Plate LXXIL In the centre of the umbilicus it has small irregularly-placed spines, 

 the rudiments of the spinous stage. It is slightly more costate and slightly more 

 carinate than substriata (type), which is represented in Plate LXXI, figs. 6 — 8, 

 and a young example, Plate LXX, figs. 6, 7. 



The acanthodes-stock. 



From acanthoses a (p. 319) it is possible to trace a genetic series of costate 

 forms which in their developments are analogous to the dominans- series. They are 

 separable from one another in direct line without much difficulty. In acanthodes 

 the spinous stage is an important feature ; in ptycta the greater part of the fossil 

 is taken up with the costate stage ; in cymatera the whole fossil is practically taken 

 up with the costate stage, the spinous stage being quite rudimentary ; in obtusi- 

 formis the whole fossil also belongs to the costate stage, but early in life the costse 

 are elaborated in a remarkable manner. 



It is always more difficult to distinguish morphological equivalents than to 

 separate the steps of a genetic series ; but the forms of the acanthodes- stock may 

 be known from those hitherto described by the following. 



In proportion to development, the forms of the acanthodes-s£oc7<; are wore umbilicate, 

 have whorls more laterally inflated, and retain their reclining ribbing longer. That 

 the combination of these features makes more and more difference between the 

 two series of the acanthodes- and dominans -stocks may be seen by comparing the 

 morphological equivalents, obtusiformis and revirescens. The separation from the 

 forms of the dominans- stock, of ptycta and cymatera, as descendants of acanthodes 

 and ancestors of obtusiformis, is of importance biologically. 



