l6o PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



Affinities: As has been pointed out above, the New Zealandian and 

 Australian species D. mantelli is closely allied to this species. It is 

 found in New Zealand, in the Pareora system of Miocene, according to 

 Hutton. 



A closely allied species seems to be : D. gabbi Pilsbry and Sharp (1897, 

 p. 470, pi. i o, f. 6, 7, 13, pi. n, f. i, 2), from the Oligocene or Miocene 

 beds of San Domingo, but it is smaller, and the number of ribs near the 

 apex is less. 



Of the European species, D. kickxi Nyst(see: Speyer, 1870, p. 199, 

 pi. 21, f. 8-1 1 ) from the Middle and Upper Oligocene seems to be the 

 most closely allied species. 



89. DENTALIUM OCTOCOSTELLATUM Pilsbry and Sharp. 



PL XXXI, Fig. 2. 



1897 D- octocostatum v. Ihering, in: Rev. Mus. Paul., v. 2, p. 266, pi. 4, 

 f. 16 (non D. octocostatum Fraas, 1867). 



1898 D. octocostellatum Pilsbry and Sharp, Man. Conch., v. 17, p. 211. 

 Shell small, slightly curved, with 6 to 8 longitudinal ribs, which are 



distant, with flat intervals, rendering the cross section polygonal. 



Diameter up to 3.5 mm. 



Remarks: This species differs from the foregoing: (i) in the small 

 size ; (2) in the smaller number of ribs ; v. Ihering gives 89, but his 

 figure shows as far as can be made out an individual that had certainly 

 not more than six ribs. In our specimens, one (the largest) has only 6 

 distinct ribs, with two very indistinct ones (on the lateral intervals). The 

 other individuals have 7 or 8. D. sulcosum, at the same size, has always 

 at least 9 or 10 ribs. 



(3) The taper in both species is different. Our largest individual of 

 the present species is 25 mm. long, the diameter at one end is 2.3 mm, at 

 the other 3.5 mm. An individual of D. sulcosum, 24 mm long, increases 

 from 2.3 to a little over 4 mm. 



The specimens from Arroyo Gio are casts, but they agree in size, curv- 

 ature and taper with this species. 



Records of specimens: Mt. of Observation, upper horizon, 6 fragments; 

 Arroyo Gio, 2 casts. 



Distribution: Jegua quemada, Suprapatagonian beds (v. Ih.). 



