52 FOSSIL REPTILIA OP THE LONDON CLAY. 



anterior border of the nuchal plate of Tri. incrassatus is smooth, slightly channeled, and 

 feebly emarginate at the middle part; the plate sends out three short, tooth-like processes 

 on each side ; the posterior angle forms a fourth process which articulates with the true 

 costal part, or end of the second rib, connate with the first costal plate {ph). The 

 first neural plate {s\, T. XVIII) is rather broader in proportion to its length than in 

 the Tri. Henrici. The second (52) and third (53, T. XVII) do not expand so much 

 behind ; the vermicular pattern is broken into distinct tubercles upon these plates. 

 The posterior lateral sides of the hexagonal neural plates are relatively longer than in 

 those of Tri. Henrici. The fifth neural plate (55, T. XVII) extends backAvards beyond 

 the fifth pair of costal plates {ph, compare with T. XVI) and articulates with the 

 sixth pair of costal plates ; but the eighth and part of the seventh neural plates are 

 superseded by ossification, extending from the seventh and eighth pairs of costal 

 plates to the median line, where those plates articulate with each other, as in the 

 Tri. Henrici and Tri. Barharce. The inner surface of the nuchal plate {ch, fig. 2, 

 T. XVIII) is divided by a transverse, slightly interrupted ridge, gently concave 

 backwards, into two nearly equal parts ; the posterior one being most excavated. 

 The inner surface of the first costal plate {ph, T. XVII and XVIII) presents the 

 prominence (c2) left by the fracture of the vertebral end of the second rib, where it 

 becomes connate with that plate, and also the oblique ridge (ci) formed by the 

 attachment of the expanded end of the first short rib. The free end of the second 

 rib (c2) is short, obtuse, depressed, convex above and flat below ; the body of this rib 

 has subsided to the level of the inner smooth surface of the costal plate, with which it 

 has become completely blended. A small portion of the body of the second vertebra 

 is preserved in connexion with the long neural arch, showing that it was slightly 

 carinate at the under surface. The breadth of the third rib (cs), where it becomes 

 connate with the second costal plate (jy/2), is rather more than one third the breadth 

 of that part of the plate ; the rib at first sinks almost to the level of the under surface 

 of the plate, and then gradually rises, increasing in breadth to its free extremity. 

 The true pleurapophysial portions of the succeeding costal plates (4, 5, 6, 7, 8, and 9, 

 T. XVII) are better defined by outline grooves, but their degree of prominence is 

 slight, except in the last pair (9), which have been liberated from the superincumbent 

 costal plates {pl%) before they reached their posterior borders. 



The minute accuracy and beauty of Mr. Erxleben's lithographs supersede the 

 necessity of further verbal description of these rare and singularly well-preserved 

 fossils. 



T. XVII gives an inside view of the almost entire carapace of the Tri. incrassatus ; 

 and T. XVIII gives an outside (fig. 1) and an inside view (fig. 2) of the fore part of 

 the carapace of the largest individual of the same species of Trionyx, from the Isle of 

 Wight, showing the nuchal plate {ch) in its natural articulation with the anterior 

 neural and costal plates. 



