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INFERIOR OOLITE AMMONITES. 



broad, somewhat flattened, the core with two faint sulci bordering a small carina ; 

 the test with a well-marked hollow carina, bordered by depressed zones. Inner 

 margin scarcely defined, inclusion slight. 



There are three marked varieties of this species : 



o. Spines in adolescent age irregularly developed, and separated from each 

 other by ribs (PI. XLVIII, figs. 16, 17; PI. L, figs. 16, 17). The adult of this 

 form is not known, but presumably it acquired a regular series of spines as in 

 the next form. 



/3. The period of irregular spines much shorter, followed by a period when the 

 spines are very strong and very fairly regular ; later the spines become irregular 

 again (PI. LVII). This form is a step in advance from a. The regular spines, 

 indicating a renewal of progression, begin earlier than they could have done in a. 



y. More compressed and quicker-coiled than a or /3. The period of irregular 

 spines is succeeded by a period of regular spines. Inner margin becomes 

 conspicuous. (This form is a descendant of « but not of /3, though in some 

 respects it is more advanced than |3.) (PI. LXV, figs. 3 — 5.) 



The form a I described in the { Quarterly Journal of the Geological Society' 

 (see synonyms) as the young of acaihthodes, not at the time sufficiently appre- 

 ciating the difference in the size and irregularity of the spines. When, however, 

 I found among some specimens of my father's collection the form called 

 crassispinata |3, 1 saw that I had made a mistake. Crassispinata (3 shows the same 

 features in its inner whorls as crassispinata a, though they do not last so long. 

 For all practical purposes, therefore, it represents the adult of crassispinata a, 

 though, strictly speaking, it is a little further in advance. Crassispinata (3 

 retained its spines throughout life, while acanthoses lost them before maturity. 

 There are also minor differences — for instance, the bifurcate ribbing of adult 

 crassispinata, and the single ribs of adult acanthodes. 



That crassispinata is not the adult of multispinata may be seen by the 

 irregularity and coarseness of the spines in the inner whorls of the former. Its 

 inner whorls (PI. L, figs. 19, 20), which are a reduced edition of multispinata 

 with the commencement of the irregularity of crassispinata, show exactly the 

 derivation of the latter from the former. 



Sonn. crassispinata is a rather scarce fossil belonging to the Concavum-zcme of 

 Bradford Abbas. 



In PI. XLVIII, figs. 16, 17, is shown an immature form of crassispinata a, in 

 which the spines are not regularly developed. Fig. 16 of PI. L gives two views 

 of a smaller specimen showing the same character, and fig. 18 is part of its 

 suture-line. Figs. 19 — 22 of the same plate illustrate the young of crassispinata. 

 In fig. 1 of PI. LVII the side view of a fine adult example is shown (its suture- 

 line, fig. 2). The regular spines begin about the commencement of the last 



