278 PROCEEDINGS OF THE ACADEMY OF 



tact 1)}' a common suture by about two-tliirds their width, tlic 

 anterior portion being separated by the last vertel)ral bone. 

 There is a great difference in the sculpture of the middle of the 

 carapace and its lateral portions. The former region is closely 

 ribbed longitudinall}', the intervening grooves being mostly un- 

 interrupted. On the middle portions of the costals the ridges are 

 more or less broken up, and distal!}' they are very delicate, form- 

 ing an isosculating pattern inclosing small pits. On the last 

 costal they retain their ridge like character. The posterior verte- 

 brals are marked by a single groove down their middle. 



Width of carajjaee at antepenultimate costal bone . 0.235 m. 

 Length from front of do. backwards . . . .095 



Width of do. costal distally ..... .048 



Len2;th of last two vertebrals ..... .037 



ExcaA'ated from the Bridger bed on the summit of Cliurch 

 Butte by the writer. A new genus, Plastomenus, Cope, was 

 described as allied to Trionyx ? No marginal bones of the cara- 

 pace except a nuchal: extremities of ribs little or not projecting 

 bcA'ond costal bones. Plastron united with carapace by one or 

 two tooth-like processes of the hyposternal bones. An anterior 

 production of the hyosternal inclosing a median fontanelle and 

 uniting by a broad suture with a clavicle (episternal). 



This genus is highly interesting as connecting more or less 

 nearly the genus Trionyx yvith the Chelydrine form Anostira. It 

 is represented bj' several si^ecies in the Bridger Eocene, all of 

 which have the sculpture of both of the genera named. The 

 plastron is ossified nearly as m Anoatirn ; but in the numerous 

 specimens obtained there Avas not one marginal bone. Never- 

 theless the strong emargination of the proximal end of the second 

 costals proves the presence of a nuchal marginal whicli does not 

 exist in Trioynjx ; if there were other marginals they must have 

 been small and inclosed in a cartilaginous margin. The first 

 costals were much shorter than tlie second, and much as in Trio- 

 nyx. A costal process of the first dorsal extends backwards and 

 was attached by sutin-e to the second costal bone, just in front of 

 its capitulum, as in Trionyx^ Chelyih'a^ etc. A singular sternal 

 bone accompanies the specimens of P. thomasii and P. triony- 

 choidcs,, but partially fractured in both cases so as to leave its 

 position uncertain. It can be nothing else than the median portion 

 of a hyosternal with the outer extremity wanting. It bounds a 

 fontanelle interiorlv, W'hich nearly- reaches the hyposternal; ante- 

 riorly it has sutures for both mososternum and clavicle. It is 

 entirely unlike anything in Trionyx ; it is thickened towards the 

 median lino and strongly sculptured externally. The hyo or 

 h3'posternal in the " bridge" indicates that portion to have been 

 long, and about as wide as is usual in Trionyx. Its free edge is 

 thin ; the sutural union with the other component bone complete. 

 The type of the genus is P. thomasii (Trionyx thomasii, Cope). 



