390 Dr. J. Allan Thomson — 



(Brocchi) occurs in the Miocene of Malta and Italy, R. bolcensis in the 

 Eocene of Italy, and R. lucida in the Recent seas of Japan. Whether 

 any of these species belong to JEtheia cannot be decided until their 

 internal characters have been examined, and the same applies to the 

 smooth Cretaceous Rhynchonellids such as R. limbata (Schlotheim). 

 It may be pointed out, however, that according to Davidson's 

 descriptions l the foramens of R. bipartita and R. lucida are not apical 

 but lie beneath the beak, so that it is quite probable that these 

 species are correctly referred to Hemithyris, while on the other hand 

 those of R. limbata and of R. bolcensis are minute, as in JEtheia, and 

 apical so far as the figures can be trusted. 



Rhynchonella patagonica, Ihering, 2 from the Patagonian formation, 

 appears to belong to JEtheia, for according to Ihering it possesses the 

 minute apical foramen, while the internal characters, so far as can be 

 judged from the figures, agree well with those of the genotype. Ihering 

 had already noticed the peculiarities of the beak, and remarked that 

 he believed that the species should be placed in a different sub-genus. 

 JEtheia patagonica is of interest in possessing numerous very fine 

 squamosal ribs, showing that in this, as in other stocks, the ornament 

 cannot be considered a character of generic value. 



b 



Fig. 2. — Beak of Hemithyris, enlarged dorsal view, showing deltidial plates, 

 a, H. nigricans ; b, H. psittacea. 



The manner in which the delthyrium of Hemithyris becomes 

 partially closed is not well understood. In most recent species, 

 including the genotype, H. psittacea, there is a large pedicle opening 

 in front of the beak, margined laterally by two deltidial plates, and 

 in front by the umbo of the dorsal valve. In some Tertiary species 

 the lateral deltidial plates unite at their bases and thus separate 

 a hypothyrid foramen from the dorsal valve, and this is the case also 

 in Basiola, Dall. 3 In all cases, however, the deltidial plates are not 

 confined to the dorsal part of the delthyrium, but are reflected down 

 the sides and unite ventrally in a plate which is joined behind to the 

 inner side of the apex, but is . free in front and separated from 

 the shell by a narrow cavity (Fig. 2). The pedicle, therefore, is 

 surrounded on three sides by deltidial plates in species of Hemithyris 



1 Ann. Mag. Nat. Hist., ser. ill, vol. xiv, p. 10, 1864 ; Geol. Mag., Dec. I, 

 Vol. VII, pp. 460-1, 1870 ; Mon. Brit. Foss. Brach., pt. ii, pp. 79-80, 1855 ; 

 Trans. Linn. Soc. Lond., vol. iv, pt. ii, pp. 168-9, 1887. 



2 Ann. Mus. Nac. Buenos Aires, t. ix, pp. 334-5, figs. 11, a-b, 1903. 



3 Bull. Mus. Comp. Zool. Harvard, vol. xliii, p. 442, 1908. 



