-398 A. 11. Noble— An Ammonite from Chalk Rock. 



more name to the nomenclature of the group. I would propose, 

 therefore, that it should be known for the present as Maiocercus 

 orbicularis. 



If the specimen is rightly ascribed to Maiocercus, the definition of 

 the genus must be modified so as to cover an " opisthosoma wider than 

 long or about as wide". The otber point of Mr. Pocock's diagnosis, 

 the forward inclination of the pleural laminae, is not affected. Further 

 diagnostic points, which may now be added regarding the dorsal 

 surface, have already been indicated. It may be remarked that though 

 the Westhoughton specimen approaches Anthracomartus in the general 

 form and proportions of its tergal plates, it suggests affinities with 

 Brachypyge in one or two respects apart from the concave borders of 

 the laminae. As already pointed out, there is some hint of a first 

 tergum still wider than the second, as it is in Brachypyge but not in 

 Anthracomartus. A less speculative point of resemblance to Brachypyge 

 (and again of divergence from Anthracomartus) is the double line of 

 suture crossing the pleural laminae. On the whole, then, the new 

 specimen, while it necessitates a slight change in the definition of the 

 genus, not only further demonstrates the distinctness of Maiocercus, 

 but tends also to strengthen its claim to stand with Brachypyge in 

 a family apart from the other Anthracomarti. 



III. — On a new species of Desmocebas from the Chalk Kock of 



Buckinghamshire. l 



By A. H. Noble, B.A., F.G.S. 



WHILST engaged on the new Geological Survey in the neighbour- 

 hood of Marlow, I had the good fortune to come across an 

 exposure of Chalk Rock yielding an exceptionally rich fauna, including 

 a single specimen of an ammonite which appeared to be new to science. 

 This ammonite was therefore shown to Dr. Kitchin, Mr. Crick, 

 Dr. Howe, and Mr. H. Woods, none of whom had seen the species 

 before or could recognize it as a form already described. The 

 discovery of new Cephalopods from the Chalk is now so rare that it 

 seems worth while to draw attention to this, although the whole of 

 the accompanying fossils have not yet been identified. 



The specimen is preserved in the form of an internal cast in very 

 hard glauconitic chalk. It shows well the sutures, but is entirely 

 septate, no part of the body-chamber being present. "When complete 

 it must have measured at least 55 mm. in greatest diameter and 

 possibly more. In the earlier formed part of the outermost whorl 

 the siphuncle is exposed to view as a narrow tube of dark-brown 

 colour with glossy surface ; in the later portions of the whorl the 

 form of the tube is not preserved, but its course is well shown. 



Desmoceras marlowense, sp. nov. (Figs. 1 and 2.) 



The shell is discoidal, with the whorls increasing slowly, the height 

 of the outer whorl being about two-fifths of the diameter of the shell. 

 The inclusion is about one-half. The umbilicus is moderately wide 



1 Communicated by permission of the Director of the Geological Survey. 



