ARIETITES CONYBEARI. 273 



Ammonites Conybeari, Escher. Neues Jahrbuch, p. 167, 1853, 



— — Studer. Geologie der Schweiz, vol. ii, p. 30, 1853. 



— — Simpson. Foss. Yorkshire Lias, p. 96, 1855. 



— — Oppel. Die Juraformation, p. 78, 1856. 



— — von Hauer. Die Cephalopoden aus dem Lias der nord. 



Alpen, p. 16, tab. ii, figs. 1 — 6, 1856. 



— — Quenstedt. Der Jura, p. 69, 1858. 



— — Terquem et Piette. Lias Inf. de 1' Est de la France, p. 28, 1865. 



— — Dumortier. Jurass. du Bassin du Rhone, t. ii, p. 22, 1867. 

 Akietites — Tate Sf Blake. Yorkshire Lias, p. 284, pi. vi, fig. 1, 1876. 



Diagnosis. — Shell discoidal, much compressed ; back tricarinated ; whorls numerous, 

 eight to ten, slightly involute and numerously ribbed; in adults ribs from forty to sixty 

 in a whorl, small, round, obliquely arcuate, and interrupted, terminating at the outer 

 carinas ; back rounded, with a large prominent obtuse keel, bounded by two deep sulci 

 and two external lateral carinas ; aperture oblong, nearly quadrate. 



Dimensions. — Transverse diameter of figured specimen 340 millimetres ; height of 

 aperture 60 millimetres ; width of aperture 68 millimetres. A specimen measured by 

 Professor Hauer was 90 millimetres in diameter ; height of the last whorl -j^j ; width 

 Yqq ; diameter of the umbilicus -f^ of the diameter of the shell. 



Description. — This is one of the most characteristic and widely distributed species of 

 the zone of Arietites Buclclandi, as shown by the table of synonyms prefixed to this 

 article. Its form in general is very persistent, and its specific characters are subject to 

 fewer variations than in most of the congeneric species. The specimen I have figured was 

 obtained from the same locality as the type drawn by Sowerby, namely, the Bucklandi- 

 beds of Salford, near Bath, from cuttings made during the formation of the Great 

 Western Railway. Another very fine large typical example, which formerly belonged to 

 me, collected at the same time and from the same locality, is now in the British 

 Museum. "It measures 18 inches in greatest diameter, and 4 inches in thickness across 

 its last chamber ; eight whorls are capable of being counted, but the innermost portion 

 cannot be made out.'' Dr. Henry Woodward, F.R.S., has kindly given me these 

 measurements. 



The adult shell consists of from eight to ten slightly involute whorls, about the width 

 of their height ; the inner whorls are nearly all entirely exposed, the turn of the spire only 

 concealing a portion of the margin. The sides are convex and costated. The ribs are 

 simple, narrow, obtuse, and slightly arched, and disappear at the outside of the lateral 

 carinas, near the dorsal sulci ; they are numerous, and increase with the diameter of the 

 shell ; in the specimen I have figured there are sixty-eight ribs in the outer whorl ; in 

 general terms the number may be said to range from forty to sixty. 



The siphonal area is broad and provided with a large obtuse prominent keel ; on 

 each side thereof is a deep sulcus bounded by an outer carina, so that the siphonal area 

 of this shell is tricarinated with two sulci between. 



36 



