150 PATAGONIAN EXPEDITIONS : 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. 



The 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, i double cast. 



Distribution: Santa Cruz (Sow.). 



78. MACTRA GARRETTI spec. nov. 



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 subtruncate, 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: Ball., 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, i 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. 

 trnncatnla (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. 



