ASYMPTOCERAS CRASSlLABRUxM. 123 



it is nearly double the distance from the basal edge of the body-cbamber that it 

 was on the sides, viz. 8 mm., making here a depressed arch. Exactly in the centre 

 of tlie periphery this arch culminates in a small sinus, after which the same 

 phenomena are repeated on the other side. 



Dimensions. 



Specimen from Claiie, in Science 

 anil Art Museum, Dublin, 

 figured PI. XXXI, fig. 2. 



Diameter of shell . . . .180 mm. 



,, umbilicus (edge to edge) . . 68 „ 



,, ,, (suture to suture) . . 40 ,, 



Height of whorl (dorso-ventral) . . 110 ,, 



Thickness at umbilical margin . . 105 ,, 



Breadth of periphery near the aperture . . 83 ,, 



All the above measurements are taken from the specimen indicated, except the 

 last, which was taken from the one represented by fig 1 on the same plate, as the 

 periphery in this was more perfect. 



Affinities. — The most nearly related species to the present one is that described 

 below — Asymptoceras Foordi, Hyatt, — but the distinction between them is strongly 

 marked. Instead of the lip-like rim in front of the aperture, which is the 

 characteristic feature of A. crassilahrum, there is in A. Foordi a broad swelling 

 here, causing the body-chamber to be produced beyond the penultimate whorl to 

 a much greater extent than is the case with the former species. Besides this, A. 

 Foordi is considerably contracted at the aperture, and the periphery is broadly 

 arched instead of being flattened, and even slightly concave, as in A. crassilahrum 

 Reinarlcs. — I drew attention on a former occasion ^ to the fact that the luimes 

 Vestinautilus andAsijiiiptoceras, as previously pointed out by de Koninck," were based 

 on errors of observation on the part of their author, Baron de Ryckholt.' Hyatt, 

 however, has restored and given currency to botli these names, first in his 

 "Genera of Fossil Cephalopods," * and more recently in his "Carboniferous 

 Cephalopods." ° 



In spite of cogent reasons that might be urged against the retention of names 

 founded upon erroneous data, the fact that de Ryckholt indicated the species to 

 which he intended his generic names to be applied may ])e held partly to justify 

 their use, though not dispensing with the necessity for detiniug the genus." It is, 



' 'Cat. Fo3s. Ceph. British Museum,' 1891, vol. ii, p. 1G7. 



2 'Faune Calc. Carb. Belg.,' 1S7S, pt. 1, p. SG. 



•'' ' Notice sur les genres Nautilus, Vestinautilus, Asymptoceras, Coya, et Tercbrirosira,' p. 4. 



* ' Proc. Boston See. Nat. Hist.,' 188.3, pp. 294, 29li. 



6 ' Geol. Surv. Texas, Fourth Annual Eeport, 1892 ' (reprint, 1893), pp. 119 and l.JO. 



* Loc. cit., p4). 77—88. 



