50 EOCENE MOLLUSCA. 



be considered as an aberrant Nautilus, connecting that genus with Aturia, and leading 

 through the Clymenida into Goniatites and Ammonites. 



The N. Parkinsoni is a discoidal shell, with regular convex sides, and an elongated 

 elliptical aperture. The specimens do not exhibit the condition of the umbilicus. 

 The septa are outwardly moderately concave, with angular lobes on each side ; the 

 dorsal lobes are very broad, somewhat concave, rounded at the extremities, and 

 reflected, although not much, towards the axis ; the lateral lobes are short, wide at the 

 upper extremities, and they taper rather suddenly ; their inferior margins are nearly 

 straight, but the superior margins present a deep sinus. The siphuncle is moderately 

 large, and is placed on the dorsal part of the septal disc, half way between the centre 

 and the margin. So far as the general character can be ascertained, the siphuncle 

 does not appear to differ from that of Nautilus, and certainly does not present any 

 analogy with the wide trumpet-mouthed funnel which distinguishes Aturia. 



This species appears to have attained a greater size than any other of the tertiary 

 Nautili ; the largest chamber in Parkinson's specimen measures seven inches in 

 breadth, and nine inches in length ; and this chamber was not the last, and conse- 

 quently not the largest. 



FamiIy^ChYMETSiiT>JE. 



Aganid^e. Pictet, Deshayes, D'Orhigny. 



Adopting the opinion of Von Buch, that the position of the siphuncle is the 

 principal, if not the only, character by which the Tetrabranchiate Cephalopods can be 

 divided into families, it becomes impossible to include those genera in which the 

 siphuncle is placed on the dorsal margin, either among the Nautilidse, in which it is 

 central or excentric, or among the Ammonitidae, in which it is placed on the ventral 

 margin. The only genera at present known to possess a strictly dorsal siphuncle, are 

 Cli/menia, Munst. (Fndosijj/wnites, Ansted), and Aturia, a genus proposed by Bronn for 

 the Nautilus Aturi, Basterot (N zic-zac, Sow.) In fact, these genera have already 

 been considered by MM. d'Orbigny, Deshayes, and others, to form a subdivision of 

 the Nautilidae, to which those authors have applied the name Aganidae, founded on a 

 genus proposed by De Montfort for a shell from the mountain limestone. This shell, 

 however, possessed a ventral siphuncle, and belonged to the genus Goniatites.* The 

 name Aganida, therefore, cannot with propriety be retained as a family name for genera 

 characterised by a dorsal siphuncle ; and I have adopted, in lieu of it, the name 

 Clgmenida, founded on Munsters genus. 



* The shell figured and described by De Montfort as Aganides is, I believe, the Goniatites sphcericus of 

 Sowerby. 



