I 5° PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
Anterior and posterior ends rounded, posterior very little more produced 
than anterior. 
Length, 48 mm; height, 36 mm; diameter, 9 (x 2) mm. 
Remarks: Sowerby calls the posterior end “subquadrate,” but his 
figure does not show this character, nor is it seen in our specimens. 
I he generic position is doubtful : I do not see the hinge in any of our 
individuals. The external form agrees completely with Sowerby’ s figure, 
but most of the specimens are larger. 
Record of specimens : Mouth of Santa Cruz River, 19 double, 9 single 
valves ; Mt. of Observation, upper horizon, 4 fragments ; Shell Gap, Rio 
Chico, upper horizon, 1 double cast. 
Distribution : Santa Cruz (Sow.). 
78. MACTRA GARRETTl Spec. IlOV. 
PI. XXIX, Fig. 9^. 
Shell thin, triangularly-subovate, compressed, a little inequilateral, 
smooth, with concentric lines of growth. Anterior end a little longer, 
rounded, posterior sub truncate, with an indistinct keel running down 
from the apex to the posterior ventral angle. 
Length, 19 mm; height, 15 mm; diameter (double), 7 mm. 
Remarks: At first I believed that this is M. indistincta of v. Ihering; 
but having sent some specimens to the author, he informs me that it is 
not his M. indistincta , but probably new, and so I describe it as new, con- 
necting with it the name of Mr. J. W. Garrett. 
This species is a true Mactra (see: Dali., 1898, p. 874), as shown by 
the hinge-teeth (pi. XXIX, Fig. 9 c>d ). 
Record of specimens : Mt. of Observation, upper horizon, ca. 40 well- 
preserved shells, and many fragments ; Lake Pueyrredon, 600' above 
base, 1 cast of right valve. 
Affinities : Among the numerous species of Mactra described by Phil- 
ippi from Chili, there is one that might be compared with M. garretti: M. 
truncatula (Phil., 1887, p. 154, pi. 27, f. 15) from Navidad. It agrees in 
the truncation of the posterior end, which is shorter than the anterior : 
but it is smaller, the anterior end is distinctly narrower than the posterior, 
and longer comparatively than in M. garretti. 
