166 Dr. J. E. Gray on the Development and Cohesion 



(PI. IV. fig. 5), the hinder pair of bones are not united together 

 behind, and each sends forth a lobe from the middle of the 

 inner side, which eventually unite in the centre line, leaving 

 a small posterior central space between the hinder ends of these 

 bones. 



This form seems common and perhaps peculiar to the family 

 Bataguridaj. I have figured the inside of the sternum of a very 

 young specimen oiKachxiga major (PI. YI. fig. 1), which has the 

 bones and lobes very slender. These parts are more developed in 

 Kackuga dentata even in the youngest state, the outside of 

 which is figured (fig. 2), and whicli has the posterior pair of 

 bones ; and in an older specimen in the British Museum this 

 is also figured from the outside. 



In Morenia and Pangshura the hinder part of the sternum is 

 ossified soonest if these vacant spaces exist in the very young 

 specimens. 



In the jouwQ Morenia Berdmorei , viboui 4 inches long, there 

 is an oblong longitudinal unossified space on each side between 

 the branches of the sternum and the margin, and an elongate 

 fom--sided space in the centre between the sutures of tlie pec- 

 toral and abdominal plates, and another rather smaller one 

 between the preanal plates. There is a series of large spaces 

 between the ends of the ribs and the marginal bone. 



In a skeleton of a half-grown Pangshura tecta there are two 

 rhombic imperfections, the one placed between the sutures of 

 the pectoral and abdominal plates, and a rather smaller one 

 between the two preanal plates. 



This form bears some relation to the bones found in a very 

 young Chelonia (PI. VI. fig. 4), where the anterior bones are 

 very narrow. The anterior and posterior lateral bones of each 

 side are separate from each other, having a simple rounded end ; 

 each of them has two digitate external lobes, extending towards 

 the margin of the dorsal disk : the anterior one has two 

 simple processes towards and uniting at the centre ; the hinder 

 one has a series of simple digitate processes extending towards 

 the centre and hinder part of the sternum. 



Another form is very peculiar ; and as yet I have only seen 

 one example, in a very young specimen oiElseya dentata (PI. V. 

 fig. 5), belonging to the Hydraspidfe, from Australia. The front 

 pair of bones, the odd bone, and the front lateral bones are all 

 united together and form a solid front half to the sternum. The 

 hinder lateral bones and the hinder pair of bones are narrow, 

 and form a margin to the hinder half of the sternum, leaving a 

 very large triangular central space. In an older specimen the 

 large naked space becomes filled up, except a very small oblong- 

 hole in the middle of the suture of the preanal plates, and a 



