50 EOCENE MOLLUSCA. 
be considered as an aberrant Nautilus, connecting that genus with Aturia, and leading 
through the Clymenid@ into Goniatites and Ammonites. 
The JV. 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 JVawtilus, and certainly does not present any 
analogy with the wide trumpet-mouthed funnel which distinguishes Aéuria. 
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. 
Family—CiLYMENIDE&. 
AcaNnID&. Pictet, Deshayes, D’ Orbigny. 
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 dorsa/ margin, either among the Nautilide, in which it is 
central or excentric, or among the Ammonitidz, in which it is placed on the ventral 
margin. The only genera at present known to possess a strictly dorsal siphuncle, are 
Clymenia, Munst. (Endosiphonites, Ansted), and Aturia, a genus proposed by Bronn for 
the Nautilus Aturi, Basterot (V. zic-zac, Sow.) In fact, these genera have already 
been considered by MM. d’Orbigny, Deshayes, and others, to form a subdivision of 
the Nautilidz, to which those authors have applied the name Aganidee, 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 Aganide, 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 
Clymenide, founded on Munster’s genus. 
* The shell figured and described by De Montfort as Aganides is, I believe, the Gontatites sphaericus of 
Sowerby. 
