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



odd bone, on the inner margin of the other branch, slightly 

 arched. 



In some genera there is only in the adult state a callosity 

 on the sides next the suture between the two middle pairs of 

 bones, as Aspilus ; in others these callosities are expanded, as 

 in Rafetus ; but generally the anal pair of bones are also 

 covered with expanded callosities, as in Trionyx. In some, as 

 Emyda, the front pair and the odd front bone are provided with 

 callosities. The Mud-Tortoises are generally without any 

 bones on the margin ; but some few bones are developed in the 

 margin of the adult animal in Emyda. 



The development of the genus Emyda has been imperfectly 

 observed. In E. punctata the margin of the disk of the young 

 is flexible, without any marginal bones. At length an oblong 

 marginal bone is developed on the front part of the hinder side 

 over the hind legs ; and afterwards a series of smaller marginal 

 bones are developed on the margin behind it. When veiy 

 young the expanded bony dorsal disk is very narrow, only 

 occupying the centre of the back, the expanded part being 

 shorter than the ribs. 



The odd front bone is rather broader than long, and separated 

 from the front pair of bones by the prominent square first ver- 

 tebral callosity ; but as the animal grows the odd first callosity 

 becomes much broader and closely united to the first pair of 

 callosities, which become wider so as quite to enclose the first 

 vertebral callosity. It is not until after this change has taken 

 place that the single anterior nuchal callosity and the two 

 hinder lateral callosities before referred to, over the hind feet, 

 are developed. At length the anterior ti-ansverse callosity is 

 united to the front of those of the first pair of ribs to form 

 the dorsal shield, and the single anterior marginal callosity fits 

 into a central notch in its front margin. 



In the young specimen the odd anterior marginal callosity 

 is not developed. When the three marginal bones before 

 mentioned are developed, then it is oblong, transverse, and 

 very small ; but it enlarges as the animal increases in size. 



In the very young specimen tlie front pair of sternal callosities 

 are small, roundish, and very far apart. They gradually increase 

 in size, being at first rounded quadrangular, rather longer 

 than broad ; but they at length spread out on the sides, and 

 are much broader than long, being broader in front than on the 

 outer side. The hinder pair of sternal callosities are always 

 separate behind. In the very young specimens the pair are 

 far apart, much longer than broad, arched on the inner and 

 straight on the outer side. As they increase in size they 

 become broader compared with their length, and closer together, 



