Cope.] ^^^ [Nov. 15, 



Trimerorhachis mesops, sp. nov. 



The greater part of the skull and vertebral column with ribs and 

 thoracic plates represent this species. The vertebral column and ribs rest in 

 a sheet of matrix whose upturned edges suggest that it contains as a support 

 a ventral armature. It also looks like a cast of a cavity left in the matrix 

 by the dissolution of the inferior body wall. The only part of the verte- 

 brae discernible without further cleaning are the neural arches. Limbs 

 not detected. The posterior border of the skull is damaged, but one angle 

 is preserved, and all of the other but the apex. The remainder is in good 

 preservation on one side or the other, and the surface has been cleaned 

 by weathering. The lower jaw is tightly closed on the upper. 



The skull does not expand posteriorly as in the T. inaignis. The pos- 

 terior border of the orbit is 4.5 times the diameter of the latter in front of 

 the angle of the mandible, and four times posterior to the line of the end 

 of the muzzle. It is thus nearly in the middle of the length of the skull, 

 and posterior to the position it holds in the T. insignis. The interorbital 

 space is nearly twice as wide as the diameter ot the orbit, while in T. 

 insignis it equals that diameter. The muzzle is therefore relatively 

 elongate, and it projects an eye diimeter beyond the line connecting the 

 anterior borders of the nostrils. The latter are large and look upwards ; 

 and the long or anteroposterior diameter equals the transverse diameter of 

 the orbit. There are no preorbital or interorbital depressions. The sculp- 

 ture is strongly marked. On the jaws it is generally longitudinal ; on the 

 supratemporal, radiating ; on the top of the front and muzzle, reticulate 

 with some predominance of the longitudinal ridge?. The sensory grooves 

 are very obscure, but are traceable on the internal border of the nostiils, 

 but scarcely posterior to them. The groove on the internal side of the 

 inferior border of the mandibular ramus is distinct. The rami are more 

 transversely expanded than in the specimens of T. insignis, but some of 

 this may be distortion due to pressure. The parasphenoid is narrow for 

 the greater part of the length. 



The T. bilobatus is known from the angles of mandibles of two indi- 

 viduals, and probably by associated remains. The corresponding parts 

 of the T. mesops are much more expanded transversely inwards, are 

 horizontal in fact, where the inner wall is in the T. bilobatus, vertical. 

 The strong internal keel of tlie latter, if represented at all in the T. mesops, 

 has an external position. 



The neurapopliyses of the vertebrse are more elevated and more deli- 

 cate than in the T. insignis, and liave the usual median longitudinal groove 

 between them on the middle line above. 



The thoracic shield is represented by a coarsely sculptured plate which 

 is but partially exposed, so that its form is as yet uncertain. 



The species is smaller than the 2\ insignia. 



3feasureme7its. mm. 



Length of skull to line of mandibular angles 136 



Width " at " " " 100 



