FIFTH, OR AGASSICERAN BRANCH. 205 
7 
Siren 
WU 
CONT) 
Fie. 34. Fie. 35. 
Var. quadragonatum, Hyarr. 
Localities. — Lyme Regis, Semur. 
A form in the Museum of Comparative Zodlogy, represented in Figures 34, 
35, closely resembles the stout variety of obfuswm, but exaggerates the char- 
acters of that form and of variety B. Though young, it has a more subquad- 
ragonal whorl than is usual in the adults of od/usum, var. B, a flatter abdomen 
and broader channels, and quite distinct pile, with a tendency to the formation 
of tuberculated genicule, though actual tubercles are not present. The diameter 
is 53 mm. ‘The outer whorl is 20 mm. on the sides, and the transverse diameter 
the same. The sides are about parallel, the abdomen flat, distinct, sub-angular, 
and genicule are present.. The abdominal part of the pile do not bend forward, 
but pass on to the abdomen and interrupt the channel ridges. The ridges are 
well marked, though not so prominent as in Twrneri. 
It is evidently a modified form of odtusum, in which the pile by a very slight 
increase in the geniculs would become tuberculated. Dumortier, in his “ Etudes 
Pal. Bassin du Rhone,” describes a specimen of obdfusum having eight or nine 
irregular nodes upon the young whorls until 20 mm. in diameter, and gives these 
as characteristic of the young of his specimens. They probably belonged to this 
variety, in which the young are more strongly marked in this way than is usual 
in other varieties of obfusum." 
There is a large specimen in the Museum of Comparative Zodlogy, from Lyme 
Regis, 500 mm. in diameter. Though considerably altered by pressure, the last 
whorl has the aspect of var. guadragonatum, and the normal form is comparatively 
slightly altered on the third qnarter of this volution. It is here only 115 mm. in 
abdomino-dorsal diameter, which is considerably less than would occur in any 
equally large specimen of other varieties. There are pseudo tubercles on the 
outer whorl, but these may in large part be due to distortion of the pile by 
VI. pl. xxv., to Asteroceras 
ue to clinologic degeneration in some species, which may, however, 
The umbilical whorls are heavily though sparsely tuberculated, and 
See also page 100, note 3, above. 
1 The similarity of Arietites stelleformis, as figured by Wahner, Unter. Lias, 
may be simply a case of parallelism, d 
belong to an entirely different genus. 
the outer whorls are, in our opinion, those of an old shell. 
