TRIONYCHOIDFA. 939 
anterior vertebrae by a small tubercle below the praezygapophyses. 
The neuro-central suture is persistent. All the vertebree except the 
first and eighth are opisthoccelous, and the articulation is ginglymoid 
between the seventh and eighth. 
SacrkaL AND CaupaL VeRteBRa.—The sacral vertebra are two in 
number, with very strong ribs suturally united together distally. 
As a rule, the sacral ribs articulate with the neural arch only ; 
Pelochelys forms, however, an exception, the ribs articulating with 
both arch and centrum, as in other Chelonians. The caudal ver- 
tebree are proccelous, with strong transverse processes which, in 
the anterior vertebree, are entirely upon the neural arch. The neuro- 
central suture persists on the anterior vertebre. They range in 
number from 14 to 19. Chevron bones are absent. 
Sxuty.—The skull, which is much depressed, is remarkable for the 
great development of three crest-like posterior processes, directed 
backwards and upwards, viz. the supraoccipital and, on each side, the 
squamosal. The orbits are moderate or small, turned obliquely 
upwards, and in some forms (especially Chitra) close to the anterior 
extremity of the skull ; they are bordered by the maxillary, the pree- 
frontal, the frontal, the postfrontal, and the Jugal. The preemaxillary 
is extremely small, single, not entering the nasal opening, and widely 
separated from the vomer, the maxillaries uniting in a median 
suture in front of the choane. The prefrontals are large and in 
contact throughout, and constantly separated from the postfrontals 
by the frontals ; the postfrontals are comparatively small, and the 
parietals join the jugals, the two latter bones forming a suture on 
the upper surface of the skull in such forms as have a broad post- 
orbital arch. The parietals, which have large descending palatal 
processes, never expand into a supratemporal roof. A zygomatic or 
temporal arch is present, but constantly rather slender, and formed 
by the jugal and the quadratojugal ; the latter bone is small, and 
in Chitra and Pelochelys separates the jugal from the squamosal. 
There is no parieto-squamosal arch. The quadrate surrounds the 
outer ear-chamber, which is completely closed, with the exception 
of a small foramen for the passage of the stapes; the tympanic 
border is formed by the squamosal above, and a rather shallow notch 
is present in the quadrate posteriorly. Two very distinct articular 
facets are present on the quadrate, corresponding with the con- 
cavities of the mandible. 
The structure of the palate is very characteristic. The vomer is 
small, separates the choanez, and is usually connected by ascending 
processes with the prefrontals. The palatines are large and form a 
median suture; they are posteriorly in extensive contact with the 
basisphenoid, which is more developed than in any other Thecophore, 
and widely separates the pterygoids. The latter bones are broad, 
with straight or convex outer borders forming more or less distinct 
wings ; they are devoid of lateral processes, and extend posteriorly 
beyond the quadrate, which they separate from the basioccipital. 
In Trionyx and Hmyda the opisthotic is produced posteriorly into 
a crest applied against the squamosal process. 
