18 G. P. Wieland — Proiostegan Plastron. 



vertebra. But, whether or not it shall transpire that my con- 

 tention is correct, it must be borne in mind that these markings 

 faced upwards. If this element had been the nuchal, whence 

 came they ? They were never in contact with any vertebral 

 processes unless such projected downwards. 



The Hyoplastron (see measurement). — This element of the 

 plastron is traversed by and marks the end of a continuous 

 heavy ridge beginning with the heavy outer edge of the com- 

 paratively narrow xiphiplastron and continuing through the 

 hypoplastron. Considered in its general outline, this ridge 

 strongly suggests the beginning of a reduction resulting in a 

 plastron like that of Sphargis. 



From its central ridge the hyoplastron, which is broadly 

 elliptical in general outline with the exception of the large 

 brachial notch, grows steadily thinner on both sides until it 

 consists simply of flat ridges marking the emergence of the 

 spines which beset it on every side except the brachial, 

 the edges being no thicker than cardboard. 



These spines narrow and thicken toward their middle por- 

 tions. Those of the inner edge may be roughly divided into 

 an 'anterior, a medial and a posterior group. The spines of 

 the anterior group, seven to nine in number, are very heavy 

 and rather short and incline to or are distinctly triangular in 

 section with the flat surface beneath. Those of the medial 

 group, six in number, are very long and slender. For their 

 last third they are crescentic in cross section, the concave faces 

 facing in pairs. The last or inferior group consists of rather 

 flat spines. There are nineteen spines in all on the right 

 interior and twenty-one on the left. Where two take the place 

 of one the general symmetry is preserved by a diminution in 

 size. 



On the outer edge following the brachial angle there are 

 three broad, heavy and rather short spines, followed by three 

 longer and more slender; the remainder of the outer hyoplas- 

 tral series being rather broad and flat. 



There is absolutely no marginal or surface marking what- 

 ever suggesting contact with an epiplastron. (See Plate II.) 



The Hypoplastron (see measurements). — This bone strongly 

 resembles the hyoplastron in general appearance, though the 

 spines are more numerous and less divided into regions, with 

 the exception of the two long posterior extensions which receive 

 the xiphiplastron between them. The right and left inner 

 series overlap considerably. Inner series of spines, right side, 

 20; left, 17; outer series, 17 in number. 



The Xiphiplastron (see measurements). — Except for the 

 slightly dactylated connection between the posterior ends of the 

 xiphiplastrals there are no spiny processes. The outer edge 



