Monks. ] 336 [March 1, 
The columella forms part of these ridges and occupies a space between 
them. 
Its primitive shape, as indicated by young specimens, and nearly adult 
sea forms, is an open triangle, or, perhaps, two rods of bone touching at 
one end, but wide apart at the base. (From young 7. carolina.) 
In some species the triangular form is lost, and in most it becomes very 
much flattened in adult age. 
Tt articulates above with the parietal, below w vith the pterygoids, in front 
with the parietal and sometimes the jugal, behind with the parietal and 
quadrate, and often forms part of the anterior margin of the foramen ovale. 
From the quadrate there extends a smal] osseous style to meet it, and in 
most cases there is a groove in the pterygoids anteriorly for its cartilagin- 
ous extension to reach the jugal 
The descending plate of the parietal generally articulates directly with 
the pterygoid in front, but the columella is between them at other points.* 
The columella in Chelonia mydas is triangular, with the front portion 
scale-like, but the back a flattened rod where it joins the quadrate. 
In young specimens it is small and slender, and is placed on the base of 
the parietal near the centre, in a deep groove. It does not touch the jugal 
nor extend to the foramen ovale in the young. 
In Aspidonictes spinifer it is thin, flat and scale-like, and either a con- 
tinuation of it or a separate bone, extends forward, and is interposed be- 
tween the parietal and pterygoid. It forms the margin of the foramen 
ovale, which, like all openings in the Trionyx skull is very large. 
Aromochelys odoratus makes a second exception to the general rule in 
specimens I have examined of the parietal reaching the pterygoids in front. 
In this case and in A. spinifer there are two bones to complete the lateral 
wall. Instead of curving upward, as in other species, they curve down- 
ward, and the anterior portion, or bone, if it proves to be separate, is firmly 
attached to the palate bone in adults. In one specimen this front part 
forms a complete column, whose hinder margin blends with the flat bone. 
The columella is flat even where it joins the quadrate and bounds the 
greater part of the front of the foramen ovale. 
In Chelydra serpentina it forms a flattened, concave triangle, and is 
joined to the quadrate, and often the jugal by two flattened rods. It is 
large, well marked, and bounds the foramen ovale. 
In Malacoclemmys palustris it is a narrow band, deeply concave, extend- 
ing from the inward process of the jugal to the quadrate, and forms part 
of the ridge in front of the foramen ovale. 
In Pseudemys scabra and Ps. concinna, it is much as in M. palustris, but 
in Chrysemys picta, Chelopus guttatus, Chelopus muhlenbergti, and tn- 
sculptus, and Cistudo clausa it is much smaller, and does not extend to the 
jugal. In all these emydes the posterior descending portion of the parietal 
forms the most of the ridge in front of the foramen ovale. 
* See Fig. 9, Ch. serpentine of Professor Cope’s articles on ‘‘ Homologies of 
Cranial Bones of Reptilia,”’ 
