ORTMANN : TERTIARY INVERTEBRATES. 135 



either the Veneridce or the Saxicavidce (Glycimeridce}. On the other hand, 

 the hinge, with the exception of the anterior part, agrees so closely with 

 that of the Cardiidce, that, comparing large species of Cardium, for in- 

 stance C. discrcpans Bast., C. laqueatum Conr., C. sulcatum Lmck., one 

 is at once struck by the close resemblance. Indeed, the hinge is identical, 

 but for the complete lack of the anterior lateral tooth in Amathusia. 



The hinge has two cardinal teeth in each valve, and one posterior lateral 

 tooth ; the lack of the anterior lateral tooth cannot be regarded as a serious 

 reason for separating this shell from the Cardiidce, since in this family 

 the lateral teeth are obsolete in other genera. 



The ligamental plates (nymphae) are very high in Amathusia, and sepa- 

 rated from the umbones by a very deep furrow, a condition that is often 

 seen in species of Cardium (for instance C. discrepans, see : Hoernes, 1870, 

 pi. 24, f. i, 2), where it is developed almost in the same degree as in 

 Amathusia. 



The pallial impression in Amathusia possesses an almost rectangular 

 upward curve posteriorly, which can hardly be called a sinus. The same 

 character, and even a distinct sinus is found in some Cardiidce, so that 

 this character also does not argue against the position with the Cardiidce. 



The most striking characters that distinguish AmatJinsia from Cardium 

 are : ( i ) the lack of the anterior lateral tooth, of which no trace is pres- 

 ent; (2) the complete absence of radial sculpture of the shell, and the 

 lack of crenulations of the lower margin. 



Record of specimens: Mouth of Santa Cruz River (just above high 

 tide) ; 2 double, 2 left valves. 



Distribution: Navidad, Chili (Phil.); Jegua quemada, Suprapatagonian 

 beds (v. Ih.) (p. 257; on p. 258, v. Ihering says that his specimens are 

 from Santa Cruz). 



Fam. VENERIDA1 Leach. 



Gen. VENUS L. 

 61. VENUS DIFFICILLIS Ortmann. 



PI. XXVIII, Fig. i"-". 

 1899 V. d. Ortmann, in: Amer. Journ. Sci., v. 8, p. 428. 



Shell thick, oblique, inflated, posteriorly a little narrowed; apex situ- 

 ated in advance of or at l / 4 of the length. Area long, occupying almost 



