part 4] JURASSIC CHEONOLOGT : LIAS. BOl 



The periphery at the beginning of the last whorl is carinate and 

 tabulate ; at the end of it, carinate-subsulcate. Body-chamber just 

 ■under half a Avhorl, specimen being presmnabl}^ complete. 



Remarks. — The smooth stage of this species being distinctly 

 keeled connects it with the keeled series of smooth Arniocerates, 

 and it is presmnably a development of the miserahile-Jlavum line. 

 The ribs being only slightly reelinate it may reasonably pass into the 

 Arnioceras-hodley i series of costate Arniocerates ; less likel}^ into 

 the next species. In the species now described the ribs are an adult 

 or old-age feature, in the costate Arniocerates a juvenile character 

 — an illustration of the law of tachy genesis. In the species now 

 described small nodes make their aj^pearance almost as soon as 

 ribs ; but in some lines of costate Arniocerates nodes are long in 

 appearing — may not appear at all. The dift^erence in the time of 

 developing nodes may be a means of distinguishing lineages in 

 costate- Arniocerates, as the time of carination seems to be among 

 the smooth forms. 



History of figured specimen. — From blue Lias Limestone 

 of Lyme Regis (Dorset) — purchased there. The test has the same 

 colour as Arnioceras Jlavicm. In the matrix is a small example of 

 what is perhaps Cymhites. 



[Since this paper was read I have obtained tbe presumed holotype 

 of AnDnonltes semicostatiis Young & Bird. It is illustrated in 

 ' Yorkshire Type Ammonites ' 1918, ])\. cxii.] 



Arxioceeas fortunatum, sp. nov. (PL XXVIII, hg. 4 ; PI. 

 XXXI, figs. 4«&4J.) 



Cf. 1883. Ammonites ceratitoides Quenstedt, 'Aminoniten d. Sclivvabisclieii Jura' 

 pi. xiii, lig. 8 (not of ' Cephalopodeii ' 1849, pi. xix, fig. 13). 



Description. — Serpenticone ; subplatygyral to substenogyral ; 

 leptogyral ; subextremilatumbilicate ; carinate-sulcate ; rursisub- 

 recticostate, parvinodate. Smooth stage to about 10 mm. diameter, 

 costulate to about 18 mm., then an injury, costate from about 

 20 mm. onwards. The costse are decidedly reelinate and nearly 

 straight on lateral area, sharply projected beyond small nodi, which 

 are situated on the periphero -lateral border. Suture-line simple. 



Remarks. — This species is much larger than any Arnioceras 

 known to me in literature ; but Mr. Tutcher has a fragment of an 

 Arnioceras from Radstock which would measure about 5\ inches 

 (140 mm.) in diameter. The species is distinct from A. hodleyi 

 (J. Buckraan) by its more reelinate, wider-spaced ribs and b}^ a 

 somewhat smaller umbilicus at the same diameter. It might be 

 suggested that the reelinate ribs were due to the injury in the 

 young stage, but it is evident that the specimen entirely recovered 

 from this, and was so little affected that it grew to an unusual size. 

 Mr. Tutcher has Radstock specimens of Arnioceras with much 

 reelinate ribs. Also Quenstedt figures a form which has the same 



