76 GEOLOGICAL SURVEY OP fEXAS. 



hypoplastral does the same into the siphiplastral. The projecting lip of 

 the epiplastral is marked off by only a slight constriction of the external 

 border from the rest of the bone. Its free border is doubly convex, 

 owing to the presence of a concavity near the middle. The posterior 

 border of the lip is very much elevated, bounding in front a strong 

 thoracic excavation. This raised border descends rapidly, and is obsolete 

 from the middle to the posterior border of the epiplastral. An angular 

 ridge bounds externally the convexity of this part of the superior sur- 

 face, disappearing opposite the point where the internal appears. The 

 depth of the lip at the internal base is equal to its anteroposterior length 

 from the same point. The inferior face of the epiplastral is flat and 

 not turned upwards, as in T. polyphemus, and the inferior surface of the 

 lip exhibits two shallow longitudinal fossse, which extend to the border. 

 The gulo-humeral scutal suture describes a segment of a circle forwards 

 and cuts across the anterior part of the mesoplastron. 



The xiphiplastron is characterized by the wide divarication of the 

 posterior angles, whose external border is continuous with that of the 

 posterior lobe of the plastron. The posterior median emargination is 

 very wide, and the projecting angle is broadly rounded. The ridge 

 which borders the external level of the lobe reaches the free border be- 

 hind, external to the median suture. The femoro-anal scutal suture is 

 nearly parallel with the border of the emargination, whence it follows 

 that it leaves the median suture at a right angle, and that the anal 

 scutum is much wider than long. The area of this scutum is thickened 

 along the anterior border and diagonally across from this to the median 

 suture. The surfaces are smooth. Scutal grooves well marked. 



Fragments of costal bones are rather thin, and their surfaces are per- 

 fectly smooth. 



Measurements. 



Mm. 



J. .-,,.■, ■. 1 , ( anteroposterior 57 



Measurements of lip at gulohumeral suture. • ■{ , . „„ 



I transverse to 



Thickness of lip at thoracic border 44 



Length of epiplastral bone (diagonal) 138 



Thickness of border of same at hyosternal suture 20 



Length of xiphiplastral (diagonal) 157 



Thickness of border of same at hyposternal suture 28 



Width of posterior median notch of plastron 122 



Depth of posterior median notch at plastron 30 



Diameters of anal scutum i anteroposterior 48 



( transverse 98 



This species is not without resemblance to the T. polyphemus. It is 

 much more robust, and the posterior plastral angle is more developed. 

 The anterior lobe of the plastron is not recurved in T. laticaudata and 

 the lip is not festooned on the border in the existing species. As com- 



