PLETTR0STEKNIDJ3. 



205 



bones and an intergular shield are developed. The pubis may 

 articulate, without sutural union, with the xiphiplastral. 



The skull and neck are unknown. The coracoid and humerus 

 (when known) are of a Pleurodiran type. 



Family PLEUROSTERNID^E. 



For the present the whole of the members of this group may be 

 classed in a single family, which will embrace the Baenidce of Cope. 

 The latter family was taken to include Platychelys {infra) and Baena, 

 Leidy *, of the EoceDe of the United States. In Baena the inter- 

 gular shield is divided, and according to Cope 2 , who remarks on its 

 generalized affinities, there are depressions on the xiphiplastral 

 which appear to have received the ischium and pubis. 



Genus PLEUROSTERNUM, Owen \ 



Syn. Megastemum, Gray i . 

 Digerrhum, Cope 5 . 



Carapace much depressed, smooth, and rounded posteriorly. 

 Neurals moderately elongated, hexagonal, with their antero-lateral 

 surfaces short. Mesoplastrals complete (fig. 45) ; entoplastral 

 relatively large ; xiphiplastrals deeply notched. No nuchal shield ; 

 vertebral shields considerably wider than long ; intergular not 

 divided ; iuframarginals present. An articulation between the 

 pubes and the xiphiplastrals (fig. 46). Coracoid much expanded 

 distally ; scapulo-precoracoid with pedunculate glenoid cavity, and 

 a right angle at the junction of the two component bones. Humerus 

 of a Pleurodiran type. 



The surface of the shell is very finely pitted. 



The pectoral girdle closely resembles that of Chelys, although 

 approximating to Hydrastis in the pedunculate glenoid cavity. 

 Frequently the 8th neural coalesces with the first suprapygal . 



1 Eep. Geol. Survey of Wyoming, 1870, p. 367. — Bcena. 



2 Eep. TJ. S. Geol. Surv. Terrs. toI. iii. p. 145 (1884). 



3 Wealden and Purbeck Eeptilia (Mon. Pal. Soc), pt. i. p. 2 (1833).— 

 Amended from Pleurosternon. 



4 Cat. Tortoises &c. Brit. Mus. p. 45 (1841). See also Supplement Cat. Shield 

 Eept. pt. i. p. 77 (1870). Original description insufficient and inexact ; the 

 name (Meg aster num) is occupied by Mulsant, Hist. Nat. des Coleopteres de 

 France, Palpicornes (1844), which may be earlier than Gray's name, which did 

 not appear till after July 1844. 



5 Trans. Amer. Phil. Soc. vol. xiv. pt. i. p. 156 (1870), 



