72 Dr. J. Allan Thomson — Brachiopod Morphology. 



2. Yentrally uniplicate (' Nucleates' of Douville), i.e. with a single 



fold in the ventral valve opposing a sinus in the dorsal 

 valve, or 



3. It may pass to the Cincta stage, in which fold opposes fold and 



sinus opposes sinus. 



By the development of a sinus in the middle of the single dorsal 

 fold, the dorsally uniplicate forms may become dorsally biplicate, and 

 by a similar process the ventrally uniplicate forms may become 

 ventrally biplicate (' Coarctatce ' of Douville). Buckman has also 

 shown how alternate multicarination and multicostation may arise 

 from the dorsally uniplicate forms, and opposite multicarination 

 from forms in the Cincta stage. It is hardly necessary to point out 

 that a dorsally uniplicate form cannot become ventrally biplicate, or 

 a ventrally uniplicate form become dorsally biplicate. 



Douville recognized the above facts more or less clearly, and used 

 them successfully to separate many forms from Terebratula and 

 Waldheimia, but unfortunately in three cases {Liothyris, Macandrevia, 

 and Neothyris) he was mistaken as to the type of folding actually 

 exhibited by the shells. His mistake apparently arose from a belief 

 that a straight front or an anterior truncation was necessarily due to 

 a retardation of the Cincta type, for he placed these three genera 

 among the 'Cincta 1 . Now a straight front or an anterior truncation 

 may be combined with dorsal uniplication, as is well shown, for 

 example, in Kitchen's figures of Jurassic Terebratula from India. 1 

 It may equally well be combined with ventral uniplication. In cases 

 where the plication is not of a pronounced type the only safe guide is 

 the course of the anterior commissure. This is plane in shells in the 

 lenticular or the Cincta stage, is arched upwards in shells showing 

 dorsal uniplication, be it ever so slight, and is bent downwards 

 in shells showing ventral uniplication. A slight tendency to 

 uniplication can always be recognized by the arching up or bending 

 downwards of the anterior commissure. 2 



Sub-family Terebraxulin-E. 



In Liothyrina 3 vitrea, the genotype, the anterior commissure 

 describes a broad, rather flattened arch, and the type of folding is 

 therefore incipiently dorsally uniplicate, and not of the Cincta type as 

 supposed by Douville, and similarly all the recent species placed in 

 that genus are either non-plicate in the lenticular stage, or are 

 incipiently dorsally uniplicate. Buckman 4 considers that the chief 

 characters of the genus (apart from its loop characters) are the 

 possession of a thin test and of four internal radiating furrows which 

 serve for the attachment of the pallial sinuses, characters seen 

 in the Chalk species assigned to Liothyrina, viz. Terebratula carnea, 



1 Palceontologica Indica, ser. IX, vol. iii, pt. i, 1910. 



2 Apparently in certain species of Terebratulina a retardation of the Cincta 

 type can set in after the shell has assumed an incipient dorsal uniplication. 



3 Liothyris, Douville, 1879, being preoccupied by Liothyris, Conrad, 1875, 

 the name was changed to Liothyrina by Oehlert in 1887. 



4 " Antarctic Fossil Brachiopoda, etc. " : Wissensch. Ergebnisse der Schwed. 

 Sudpolar Exped., 1901-3, Bd. iii, Lief, vii, pp. 26-7, 1910. 



