48 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
of certain other forms, is overlapped dorsally by the parietals and 
ventrally by the parabasal. The middle portions of the periotics 
are contiguous in the mid-dorsal region with the supra-occipital 
cartilage, and posterior to this they diverge, forming two processes, 
the occipital condyles, thus suggesting the probable incorporation 
into this region of the exoccipital elements. 
On the outer edge of the ventral surface of each periotic is the 
large fenestra ovalis into which the large oval operculum fits. On 
its antero-lateral border, the periotic is overlapped by the paraquad- 
rate, while the trabeculo-quadrate isthmus and the posterior end of 
the trabecula are contiguous to its anterior border. The inner 
edges of the periotics are connected posteriorly by the basi-occipital 
cartilage, which they partially invest laterally. 
The quadrate or suspensorium consists of a broad, fiat, bony head 
articulating with the rounded end of Meckel’s cartilage, and a taper¬ 
ing cartilaginous neck which is closely wrapped by the anterior 
portion of the paraquadrate. 
The mandible and hyoid apparatus are firmly held in place by 
three ligaments : the ligamentum hyo-suspensoriale, from the inner 
anterior portion of the neck of the quadrate to the distal end of the 
hyoid ; the ligamentum mandibulo-hyoideum internum, from this 
latter point to the postero-ventral angle of the angulare ; and the 
ligamentum mandibulo-hyoideum externum, to this latter point from 
a point on the anterior edge of the hyoid some distance below the 
attachment of the other ligaments. A membrane extending from 
the skull to the distal end of the hyoid is also present, arising partly 
from the operculum. It thus resembles the condition in Necturus 
as described by H. H. Wilder (: 03) rather than the condition which 
Huxley (’74) describes in that animal. 
The paraquadrate is a thin bony plate, convex on its outer surface 
and deeply grooved on its inner surface. In the anterior portion of 
this groove lies the quadrate, while the posterior part is in contact 
with the antero-lateral portion of the otic capsule. 
The operculum is large as compared with the size of the capsule 
and is cartilaginous. It lias a short columella, but this is not joined 
to the paraquadrate. 
The brain case. — Several dermal bones and the chondrocranium 
enter into the formation of the brain case. The roof is formed by 
the frontals and parietals, the floor by the parabasal, while the 
