ORTMANN I TERTIARY INVERTEBRATES. 6 1 



mouth toward the posterior periphery. Mouth opening subcentral, sur- 

 rounded by a floscelle. 



Diameter about no mm. Height about 28 mm. 



Remarks: All three specimens are imbedded in a hard, reddish, coarse 

 grained sandstone, and are poorly preserved, which refers especially to the 

 apical system, the details of the ambulacra, and the mouth. The anal de- 

 pression, however, is seen in all three individuals, and forms the most 

 striking character of the genus Cyrtoma of McClelland (1840, p. 185), al- 

 though the original diagnosis of the genus does not bring out this char- 

 acter sufficiently. It runs thus : " Disc oval and thin, arched to the apex ; 

 ambulacra petaloid, and either broad and flat, or more elevated, and 

 placed on narrow ridges radiating from the apex to the disc. The two 

 posterior ambulacra are closer together than the others, with an interme- 

 diate dorsal ridge leading to a dentate anus, and a depression or hollow 

 between the latter and the disc. Inferior surface flat, mouth small and 

 central, with five clavate ambulacra prolonged to margin." 



It would be impossible to recognize the genus from this diagnosis ; but 

 the figures given by McClelland clearly establish its position with refer- 

 ence to the genera Echinobrissus, Cassidulus, etc., and there is no doubt, 

 that it is identical (see Zittel, 1880, p. 529) with Stigmatopygus of d'Or- 

 bigny (1860, p. 331). According to d'Orbigny this genus comes near 

 Cassiduhis, and differs chiefly in the peculiar shape of the anal depression. 

 Desor (1858, p. 296) compares it with Echinanthtis. Of the floscelle sur- 

 rounding the mouth a few traces are preserved in one of our specimens. 



Record of specimens : Lake Pueyrredon, base; 3 sp. 

 , Affinities: The genus Cyrtoma (= Stigmatopygus] is known from the 

 Cherra Poonji beds of British India (Assam Range), which are evidently 

 of the age of the Arialur group (Senonian) of southern India, where the 

 genus has also been found (see: Medlicott and Blanford, 1879, part i, p. 

 280, ff., and part 2, p. 688, f., and Stoliczka, 1873, p. 27). It has also 

 been found (Stigmatopygus] in the upper Cretaceous (Senonian) of France 

 (Angouleme). Our species from the Tertiary beds of Patagonia extends 

 considerably the range of this genus in time. It differs at the first glance 

 from all the known species in the much more depressed -test, and in the 

 lack of a ridge between the apex and the anal depression. The discovery 

 of this genus, so far belonging exclusively to the upper Cretaceous, is one 

 of the most important palaeontological results of Mr. Hatcher's explora- 

 tions in Patagonia. 



