H 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
The type is apparently a left valve and the right valve is probably very 
similar with subequal ears and without a byssal notch, since there are about 
twenty such valves in the collection, and there are no others of different 
form that could be right valves of this species. The form is like that of 
the group for which Meek proposed the name Entolium, but it lacks the 
diverging “ teeth” characteristic of that subgenus. Similar forms are not 
uncommon in the Jurassic and Cretaceous, such as Pecten operculiformis 
Gabb from the Lower Cretaceous Horsetown beds of California, which, 
however, has broader ears and no radiating striae. 
Locality and position . — From the Ammonite (Belgrano) beds at mouth 
of canon four miles east of Lake Pueyrrydon, and ten miles east of the 
lake. The figured specimen is from the latter locality. 
Pecten octoplicatus sp. nov. 
PI. IV, Figs. 2 and 3. 
Shell small, ovately subtriangular, conspicuously inequivalve, with eight 
strong radiating ribs on each valve. Right valve very convex, with prom- 
inent narrowed beak ; anterior ear elongate-triangular, with a deep byssal 
notch beneath it ; posterior ear much smaller and inconspicuous ; ribs very 
prominent, about as broad as the interspaces, subangular, each bearing 
several obscure radiating lines and crossed by numerous fine concentric 
lines. Left valve much less convex, with subequal, rather broad triangu- 
lar ears ; ribs more rounded, much less elevated and relatively more nar- 
row ; the radiating and concentric lines also less conspicuous than on the 
right valve. 
Height of an average specimen from beak to base 9 mm.; greatest 
length at right angles to above measurement 8 mm.; convexity of right 
valve about 4 mm., of left valve about 2 mm. 
Locality and position . — Abundant in the Ammonite (Belgrano) beds at 
mouth of canon four miles east of Lake Pueyrrydon and represented by 
three left valves apparently belonging to the species from the same ho- 
rizon, ten miles east of the lake. 
Avicula (Oxytoma) tardensis sp. nov. 
PI. IV, Figs. 6 and 7. 
Shell small, obliquely ovate, inequivalve ; length of hinge line not quite 
equal to height of shell ; beaks rather prominent, extending beyond the 
