THE CRASSA-STOCK. 347 
costate stage the whorls are flatter-sided, altogether thinner, and less ornamented. 
The ribs, which are not so strongly marked, do not show the “ biplication”’ so 
conspicuously in alternata a as in biplicata; but in alternata B, in which it is not 
hidden by the overlapping whorl, the biplication is very noticeable. The 
alternation of a well-marked and an obscure rib is a noticeable feature on the last 
part of the whorl of alternata. In addition to the points noted above, the more 
compressed, more involute shape, and the more marked inner margin separate 
alternata from biplicata. 
The specimen depicted as alternata a was one of the forms which I regarded 
as Harp. adicrwm in a paper on Inferior Oolite Ammonites (see syn.). Although 
it agrees to a certain extent with Waagen’s fossil, so far as the spinous stage is 
concerned, yet in the costate stage its coste are far more numerous: its whorls 
are also thicker. 
Sonn. alternata is a rare fossil from the Concavwm-zone of Bradford Abbas. 
The example shown in PI. LXXVII, figs. 3—5, possesses the test, but its ner 
whorls have suffered somewhat in cleaning. The young shell depicted in PI. 
LXXVI, figs. 7—9, shows irregularity of costz and biplication very effectively. 
It is an excellently-preserved specimen. 
The crassa-stock. 
This stock contains some of the most characteristic Sonninize of the Concavum- 
zone. Some of the spinous species I had at one time confounded’ with Am. 
adicrus, Waagen ; but this was an error committed when I possessed only a few 
specimens of the genus compared with my present series. J need scarcely point out 
the many differences between these forms and adicrus—differences of whor]-shape, 
spinosity, costation, and suture-line, none of which can be disregarded. 
The forms of the crassa-stock are characterised by possessing nearly upright 
ribs even during the spinous stage, a feature which brings them into connection 
with the dominans-stock ; but the species of the crassa-stock may be distinguished 
therefrom by being thicker and more spinous in proportion to the size of the umbilicus. 
There is one exception to this which I shall notice in its place (p. 356). 
The members of the crassa-stock may be further subdivided into two series, 
namely, species in which the costate stage is fairly developed, and species in 
which the costate stage is very short. In the latter series the shortness of the 
costate stage brings the smooth stage into close connection with a well-marked 
1 ‘Quart. Journ. Geol. Soc.,’ vol. xxxvii, 1881. 
