﻿340 



INFERIOR OOLITE AMMONITES. 



rounded carina. Inner margin smooth, well-marked. Inclusion about two-fifths. 

 Umbilicus regularly graduate, centrally costate. Suture-line with a rather broad 

 superior lateral lobe, of which the lateral lobules are not very prominent. 



This is a well-known Bradford Abbas fossil, and specimens have lain in my 

 cabinet for more than twelve years under the name parvicostata. It is undoubtedly 

 a further retrogression of costata a, and has just the same peculiar, compressed, 

 angular whorls ; but its thinner form and deficiency of ornament, especially when 

 adult, render it easily distinguishable from that species. 1 



Among the examples of parvicostata there is no very noticeable variety. 

 There is a slight difference in costation and in thickness, and the inner margin, 

 which is usually upright, may be somewhat sloped. It should be noted that 

 the inner margin is flat, with a rather angular upper edge, that the whorls are 

 nearly flat, and the centre without spines, because there are forms of Sonninia, not 

 unlike parvicostata in general appearance, in which these characters are different, 

 the inner margin subconvex, the whorls somewhat laterally-gibbous, the centre 

 spinous. In my opinion they belong to another genetic series. 



Sonn. parvicostata is nearly the morphological equivalent of Sonn. substriata, 

 but is not quite so retrogressive a form. Its umbilicus is larger and more 

 concentric, its ventral area flatter and more carinate, especially in adults, which 

 also show irregular though rather obscure ribs not found in substriata. 



This is a common and characteristic fossil of the Concavnm-zone of Bradford 

 Abbas. 



In PL LXXV, fig. 3, is represented a portion of the side-view of an adult shell ; 

 an outline of its front view is given in fig. 4 ; and a suture-line from another 

 specimen is shown in fig. 5. In PI. LXXIV a specimen illustrating the involute 



1 I mentioned (page 339) in connection with Sonn. costata the resemblance of that species to 

 Dumortieria grammoceroides. During the progress of this sheet through the press a chance find showed 

 that the superficial resemblance between the species of the costata-stoc\i and Dumortieria was more 

 remarkable tliau I had imagined. 



In a case of specimens belonging to the collection made by Mr. Darell Stephens, F.G.S., I found 

 an Ammonite which had escaped my notice in previous examinations. Its resemblance to Sonninia 

 struck me, but sufficient of the suture-line was exposed to show that it could not belong to this genus. 

 At first its generic position puzzled me, but clearing the test from the suture showed a species of 

 Dumortieria — evidently an involute, senescent development of Dum. grammoceroides. The specimen 

 is extremely interesting in connection with my surmise that there ought to be descendants of grammo- 

 ceroides (page 200) ; but it is chiefly remarkable for the fact that while grammoceroides and costata are 

 superficially alike, this development of grammoceroides is curiously like the development of costata. 

 In general shape, and in ornamentation obsolescent with age, the two forms are very similar ; but the 

 development of grammoceroides is less carinate, more compressed, and has reclining ribs in its inner 

 whorls. The suture-line, especially the dependent inner portion, is the striking feature of contrast. 



I shall give a figure of this species when opportunity presents, and when I have fully investigated 

 certain matters in connection with other species. It appears to me that this chance discovery will 

 alford a very interesting illustration of complicated convergence in regard to several species. 



