156 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



Length, 24 ; height, 15 ; diameter, 14.5 (double) ; (cast from Las Salinas). 



It seems, however, that this species grew much larger: specimens from 

 Mt. of Observation and Shell Gap, only in fragments, indicate a length of 

 about 60 mm, by about 30 mm in height. 



V. Ihering says that his figured specimen measures : length, 37 mm ; 

 height, 23 mm; but the figure itself, which is said to be natural size (i/i), 

 is only: length, 23 mm; height, 15. 



Record of specimens : Mouth of Santa Cruz River, i right valve ; Las 

 Salinas, i double cast; Mt. of Observation, upper horizon, 4 double, i 

 right, i left valve, 7 casts ; Shell Gap, Rio Chico, upper horizon, i double 

 cast; Lake Pueyrredon, 600' above base, i cast of right valve. 



Distribution: Patagonian beds of Santa Cruz (Phil., v. Ih.). 



Affinities: Although this genus is not rare in Eocene deposits, none of 

 the Eocene species agrees so well with M. patagonica, as M. peroni Cossm. 

 & Lamb, from the Oligocene of Switzerland (see: Kissling, 1896, p. 45, pi. 

 4, f. 4, 5), especially as regards the general form and position of the im- 

 pressions on the cast. M. peroni, however, is smaller than our species. 



On the other hand, our species resembles also the Pliocene and recent 

 type-species of the genus Pholadidea, P. papyracea (Sol.) (see: Wood, 

 1856, p. 298, pi. 30, f. 10, Woodward, 1854, pi. 23, f. 20, and Philippi, 

 1851, p. 128, pi. 2, f. 3), and it is possible that it belongs to this genus, 

 which fact would point to a much younger age (Neogene). 



87. MARTESIA PUMILA Ortmann. 



PI. XXX, Fig. 6"-*. 

 1900 M. p. Ortmann, in: Amer. Journ. Sci., v. 10, p. 371. 



This form resembles M. patagonica, but it is much smaller, the callous 

 plate of the anterior margin is very small, and the ribs of the anterior 

 part of the valve form a very obtuse angle with the lines of growth of the 

 posterior part. The radiating furrow is narrower, and runs more inclined 

 posteriorly, so as to render the posterior part of the shell smaller in com- 

 parison with the anterior. 



The ribs of the anterior part are less in number, and the gaping of the 

 anterior margins is very slight : indeed, in the cast (and we possess only 

 casts with slight traces of shell remaining), the anterior end seems to be 

 closed : only the lowermost rib does not run parallel to the lower anterior 



