CHELONIA. 49 



by the extension of ossification from the costal plates into the dermal cartilage above 

 the neural spines. The eighth neural plate is wholly obliterated or superseded by a 

 similar encroachment and union of the eighth pair of costal plates (pis). Almost the 

 same modification is represented by Geoffroy in the carapace of the Tri. jEgyptiacus ■* 

 but the general proportions of the carapace of the Tri. IJenrici are more like those in 

 the Tri. suhplanus, in which the eighth neural plate exists in the interspace of the 

 eighth pair of costal plates, as it does likewise in Tri. Maunoir. 



All the exterior surface of the expanded parts of the neural spines and ribs is 

 roughened or sculptured with a moderately fine vermicular pattern, the undulatory 

 grooves having a tendency to a concentric arrangement at the peripheral surface of the 

 carapace, and in general passing uninterruptedly from one costal plate to another : the 

 pattern is effaced from about one third of an inch of the border of the carapace, which 

 presents a surface like that of a coarsely-woven cloth. The extreme border is rather 

 suddenly bevelled or rounded oif from above downwards, and is thinner than the 

 border of the costal plates that articulates with the neural plates. The natural extent 

 of the ordinary narrow extremities of the ribs cannot be determined from the present 

 specimen of the Tri. Ilenrici ; they form the usual slight relief along the middle of 

 the smooth under surface of the connate costal plates ; and do not subside at any 

 part of their course to the level of the under or inner surface of the plate. 



Plate 6, fig. 1, shows the upper surface of the carapace of the 'Tri. Henrici, half 

 the natural size. 



Fig. 2, in outline below, gives the curve and degree of transverse convexity across 

 the middle of the carapace. 



Fig. 3, the nuchal plate of apparently the same species of Trioni/x, half the natural 

 size. 



These specimens were discovered by the Marchioness of Hastings in the Eocene 

 sand at Hordwell, and are preserved in her ladyship's Museum at Efford House, near 

 Lymington, Hampshire. The species is dedicated to her ladyship's husband, Captain 

 Henry, R.N. 



In the figure of the carapace of the Trionyx {Tri. subplanus) in Cuvier's pi. xiii, 

 fig. 5, ' Ossemens Fossilcs,' tom. v, pt. ii, the costal plates do not bear the same numbers 

 as the corresponding neural plates ; the anterior costal plate is marked a\, whilst the 

 corresponding neural plate is 52 ; the rib or pleurapophysis of the first dorsal vertebra, 

 which is marked c\, is short, and is applied to the under and fore part of the second 

 rib which supports the first costal plate. In PI. 6, the dermal ossifications of the 

 carapace bear the same letters and numbers as the homologous parts in the previous 

 plates, and in the woodcut, fig. 1, p. 3. 



* Ibid., pi. 2, A. 



