i6 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
is broken off and some of the hard, sandy matrix still clings to a part of 
the surface. 
The species may be compared with G. alpina Pictet and Roux, 1 which 
is almost as large and somewhat similar in form, but is a straighter shell 
and has a more distinct anterior wing. Gervillia dentata Krauss 2 from 
the Uitenhage beds of South Africa is another large species that has some 
general resemblance to this form, but it is not so thick shelled, the beak 
is not so pointed and the shell is straighter. 
Locality and position. — From the Ammonite (Belgrano) beds at mouth 
of canon four miles east of Lake Pueyrrydon. 
MyTILUS (?) ARGENTINUS Sp. 110V. 
PI. IV, Fig. 4. 
Shell small, elongate-ovate, moderately convex ; beaks prominent, 
slightly incurved ; dorsal margin slightly convex without definite posterior 
angulation ; ventral margin nearly straight, posterior end regularly and 
broadly rounded ; surface marked only by very fine growth lines and con- 
centric wrinkles, which are inconspicuous except when magnified. 
Length of an average specimen from beak to base 8 mm.; greatest 
breadth 5.5 mm.; convexity of the two valves about 6 mm. 
This little species has almost the form of a Crenella, and some obscure 
radiating lines on a weathered specimen increased the resemblance and 
led to its reference to that genus, when the collection was first examined. 
Well-preserved specimens, however, show no radiating sculpture. 
Locality and position. — The figured type and four other specimens are 
from the Ammonite (Belgrano) beds at mouth of canon four miles east of 
Lake Pueyrrydon, and one specimen from the same horizon ten miles east 
of the lake. 
Lithophagus sp. 
This genus is represented in the collection by a single small flask-shaped 
burrow in a fragment of oyster shell from the Ostrea horizon (Gio beds) 
400 feet below the Ammonite layer at the mouth of the canon four miles 
1 As figured by Pictet and Campiche, Terrain Cretace de Sainte Croix, pi. 155, figs. 2-4. 
According to these authors the species includes the Cretaceous forms figured by Sowerby (Min. 
Conch., pi. 511) under the name G. aviculoides but not belonging to that Jurassic species. 
2 Nova Acta, Vol. 22, p. 458, pi. 50, figs, la-ic. 
