388 Dr. J. Allan Thomson — 



it is more probably a spinous development of Hemithyris nigricans." l 

 Chapman 2 also speaks of Acanthothyris squamosa (Hutton), and here 

 the same argument applies ; this species, which is imbricate in 

 ornament, presents all the characters of Hemithyris, to which genus 

 it should be assigned. It is probable that Hemithyris nigricans, 

 which is coarsely ribbed and not imbricate, is a catagenetic develop- 

 ment of a coarsely ribbed, imbricate, Oamaruian (probably Miocene) 

 species, not yet named, which differs from H. squamosa in its much 

 coarser ribs, while H. doderleini may be a spinous (anagenetic) 

 development of the same imbricate ancestor. 



If spinosity is not in itself a generic character, it is still less of 

 sub-family value. Schuchert has used it to separate Acanthothyris 

 under the Acanthothyrinse from the majority of smooth or ribbed 

 Mesozoic and later Rhynchonellids which are placed in the Rhyncho- 

 nellinse. Without doubt Acanthothyris is a spinous development of 

 one of the Jurassic ribbed stocks distinguished byBuckman, and some 

 other characters must be employed if the Rhynchonellidse are to be 

 subdivided into natural sub-families. 



Types of Folding. 3 



Of the characters liable to variation in the family, one to which 

 few exceptions exist is the more or less strong dorsal uniplication, 

 which finds its greatest expression in Rhynchonella itself. In Hemi- 

 thyris this type of folding is present but not always strongly marked, 

 some of the species being almost non-plicate, but the southern fossil 

 forms of the H. nigricans series are often strongly folded. Biplication is 

 not known amongst Rhynchonellids, with the exception of Rhynchonella 

 salpinx, Dall, 4 an American Eocene form which is narrowly biplicate 

 with two additional lateral folds. This species has not only the 

 type of folding characteristic of Dallina, but has also Dalliniform 

 beak characters, and as the internal characters are unknown it is 

 more than likely that it is not a Rhynchonellid, but a Terebratulid 

 in which the pores have been filled up by an unusual process of 

 fossilization. 



Ventral uniplication is well marked in the Triassic Norella, Bittner, 

 and is incipiently displayed by the Recent Cryptopora, Jeffreys. Dall 

 has described under the name of Hemithyris strebeli 5 a mid-Pacific 

 abyssal shell which shows clear ventral uniplication, and therefore 

 cannot belong to Hemithyris, but must be placed in a new genus. 



Neorhynchia, gen. nov. 

 Genotype Hemithyris strebeli, Dall. 

 Yentrally uniplicate, shell thin, broad, smooth. Beak hypothyrid. 

 Dorsal valve with a thin, thread-like septum, no hinge-plate. Ventral 



1 S. S. Buckman, "Antarctic Fossil Bracliiopoda collected by the Swedish 

 South Polar Expedition": Wissensch. Ergeb. Schwed. Siidpolar -Exped., 

 1901-3, Bd. iii, Lief, vii, p. 11, 1910. 



2 P. Chapman, Australasian Fossils, Melbourne, 1914, p. 167. 



3 Cf. J. A. Thomson, " Brachiopod Morphology: Types of Folding in the 

 Terebratulacea " : Geol. Mag., Dec. VI, Vol. II, pp. 71-6, 1915. 



4 Trans. Wagner. Free Inst. Sci. Philadelphia, vol. iii, pt. vi, pp. 1535-6, 

 pi. lviii, figs. 5-7, 1903. 



5 Bull. Mus. Comp. Zool. Harvard, vol. xliii, p. 441, 1908. 



