I2O PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 



Affinities: There is no doubt that this species is the descendant of P. 

 geminatus. Closely allied is: P. tenuicostatus Hup. (Philippi, 1887, p. 

 210, pi. 47, f. i, and Moericke, 1896, p. 580, pi. 12, f. 13-16) from the 

 Navidad beds of Tubul, Chili, but in P. tenuicostatus the byssal sinus is 

 less developed, and the ears are not so unequal. 



P. vidali Phil. (1887, p. 212, pi. 47, f. 5) from the Pliocene of Coquimbo 

 has stronger, more distinctly fasciculate, and less numerous ribs ; the 

 right valve is not so flat. 



Fam. MYTILID^. Flem. 



Gen. MYTILUS L. 

 42. MYTILUS CF. CHORUS Molina. 



PI. XXV, Fig. I - 6 . 

 1843 M. ch. d'Orbigny, Voy. Amer. men, v. 5, p. 647. 



1858 M. ungulatus Reeve, Conch, icon., v. 10, pi. 2, f. 4. 



1887 M. chorus Philippi, Tart. Quart. Verst. Chiles, p. 202. 



1889 M. ch. Clessin, in: Martini & Chemnitz, Syst. Conch. Cab., v. 8, p. 



65, pi. 5, f. i, pi. 9, f. i, 2. 

 1897 M. c f- c h- v - Ihering, in: Rev. Mus. Paul., v. 2, p. 232, pi. 9, f. 55. 



Shell elongated-oval, thick, smooth, except for the growth lines ; apex 

 acute ; dorsal margin slightly curved, about half as long as the total 

 length of the shell, forming a very obtuse angle with the posterior mar- 

 gin. Ventral margin almost straight. 



Meastirements (of Cape Fairweather specimen) ; Length, 1 1 1 mm ; 

 height, 63 mm. 



Remarks: Our specimens from Rio Chalia are very poor; they are in- 

 complete casts, with a few fragments of the shell remaining. As far as 

 can be made out, they agree in shape well with M. chorus, and since this 

 species is also mentioned by v. Ihering from the Patagonian beds, it 

 seems quite probable, that we have to deal with this species. One of the 

 casts from Cape Fairweather shows well the external form, and is indis- 

 tinguishable from the rest. 



Record of specimens: Upper Rio Chalia, remains of 6 valves; Cape 

 Fairweather, 2 casts. 



Distribution: Living : Chili. Fossil ': Santa Cruz, Patagonian beds (v. 

 Ih.); Quaternary of Chili, and doubtfully in the Navidad beds (Phil.). 



