400 



A. H. Noble — An Ammonite from Chalk Rock. 



The specimen is preserved in the Museum of Practical Geology, 

 registered 25456. 



Remarks. — It must be mentioned that the appearance of rapid 

 modification to a less involute state, presented by the last-formed part 

 of the last whorl as seen in Fig. lb, is entirely due to the imperfect 

 preservation of that part of the specimen. There also the peripheral 

 region of the cast has become broken away, and is restored in the Figure 

 by a dotted outline. It is not possible to say how much of the 

 septate part of the specimen is missing, and the length that might have 

 been attained by the body-chamber is, of course, also unknown. 

 Although uncoiled or partially uncoiled ammonitoids, such as Hetero- 

 ceras, Scaphites, Crioceras, and Baculites, are fairly common in the 

 Chalk Rock, few species which are unmodified in this manner have 

 been recorded. Mr. H. "Woods has described two, Pachy 'discus per ampins 

 and Prionocychis neptuni, 1 and Messrs. Chatwin & Withers have 

 recently identified a new species, Puzosia curvatisulcata, from the 

 Waterworks Quarry, Marlow. 3 



Fig. 2. Desmoceras marloivense, sp. nov. Part of suture-line, x 2. 



The specimen here described is referred to the genus Desmoceras 

 principally on the evidence of the suturedine, which is well shown. 

 The lobes are trifld, the first lateral being of about the same length as 

 the ventral. The saddles are similar to one another, and each one is 

 divided into two parts. A certain degree of similarity is shown to 

 Desmoceras pyrenaicum, de Grossouvre, from the Middle Senonian 

 (Santonian) of Sougraignes, Aude. 3 The suturedine is almost identical 

 in the two forms, and they are alike in the presence of constrictions. 

 These, however, are more oblique in their course and take a stronger 

 forward bend on the peripheral area in the French species. 



D. pyrenaicum is considerably more involute, and the outline of 

 the wdiorl also differs ; the peripheral zone is more convex and the 

 umbilical zone less steep than in the Euglish form. 



My best thanks are due to Dr. Kitchin and Mr. Clement E,eid for 

 their kind assistance in the preparation of this paper. 



1 H. Woods, "The Mollusca of the Chalk Eock," Part I: Q.J.G.S., 

 vol. hi, pp. 77, 79, 1896. 



2 C. P. Chatwin & T. H. Withers, " Contribution to the Fauna of the Chalk 

 Eock" : Geol. Mag., Dec. V, Vol. VI, p. 66, 1909. 



3 A. de Grossouvre, " Eecherches sur la Craie Superieure," 2 e Partie : Mem. 

 Carte Geol. France, p. 168, pi. xxv, fig. 2, and pi. xxxvii, fig. 9, 1893. 



