ORDER CHELONIA. IO97 



rounded ; the nuchal is but slightly emarginate ; and the intergular 

 shield is double. The surface of the carapace is usually marked by 

 flutings. Some at least of the Chelonians from the Kimeridgian of 

 Hanover which have been described under the preoccupied name 

 of Stylemys may be included in Plesiochelys, and this genus is also 

 represented in the English Wealden. 



Allied Chelonians, from the Kimeridgian of Hanover, described 

 under the name of Chelonides (likewise preoccupied), not improbably 

 indicate forms connecting Plesiochelys with the undermentioned 

 Hylceochelys. In one specimen referred to the type species of Chel- 

 o?iides, there are but seven neural bones ; but the vertebral shields 

 are narrower than in typical species of HylcEOchelys. If these forms 

 are really entitled to rank as a distinct genus they require a new 

 name. A skull said to have been associated with the shell of the 

 type species of Chelonides has the temporal fossae roofed over by 

 bone, while the palatines are described as meeting in the middle 

 line after the manner of many existing Pleurodira. From the occur- 

 rence of Plesiochelys and the undermentioned Hylceochelys in the 

 Wealden, and the apparent rarity of the Acichelyidce in the same 

 beds, it is probable that the members of this family were of fresh- 

 water habits. 



It is probable that the genus Parachelys of the Lithographic Lime- 

 stone (which was considered by Professor von Zittel to be indis- 

 tinguishable from Acichelys) is really identical with Plesiochelys ; in 

 which case the former name should supersede the latter, and the 

 family name Parachelyidce replace Plesiochelyidce. The humerus of 

 this genus resembles that of recent Pleurodira, and is widely different 

 from that type of humerus which appears to be referable to the 

 Acichelyidce. The number of the phalangeals in the digits of the 

 manus of this form (and also in Idiochelys) is 2, 2, 3, 3, 3 ; and this 

 presents a decided approximation to the existing Pleurodiran genus 

 Pelomedusa, which differs from all other freshwater forms now living 

 in that there are only two phalangeals in each digit. 



The name Hylceochelys has been proposed for an allied genus of 

 Chelonians typically represented by the so-called Pleurosternum 

 latiscutatum of the English Purbeck, which as shown by a shell 

 from the Wealden preserved in the British Museum, belongs to the 

 present family. This genus is distinguished from Plesiochelys by the 

 much wider vertebral shields of the carapace, in which the width 

 may exceed three times the length ; and also by the narrower and 

 diamond-shaped entoplastral bone ; as well as by the circumstance 

 that the neural bones, at least usually, do not join the suprapygals, 

 and are generally interrupted in the middle of the series ; while the 

 hypoplastral bone is relatively shorter. Both in this and the pre- 

 ceding genus there may be a vacuity in the middle of the plastron. 



