370 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



are very thick and strongly built, forming the main lateral braces of the head and being 

 supported in turn by wide transverse extensions of the braincase, formed distally at least 

 chiefly by the stout sphenotics and pterotics. The broad flat occiput evidently afforded 

 insertion for thick axial muscles. It is braced posteriorly by coalescence with the expanded 

 neural arch of the first vertebra. The basioccipital condyle is tightly appressed to the first 

 centrum, while the exoccipitals meet the anterior zygapophyses of the first vertebra in 

 dentate sutures, which have replaced the lateral occipital condyles. 



The much expanded pectoral girdle is tied to the rigidly braced occiput by the chain 

 of small bones already mentioned and by a long underlying rod, which may be either an 

 ossified tendon or the medial process of the posttemporal or both. In this feature Astro- 

 scopus is closer to Crapatulus of the Leptoscopidae than to Uranoscopus {cf. Starks, 1923, p. 

 289). At its antero-inferior end the pectoral girdle is tied by ligament to the greatly ex- 

 panded lower end of the hyoid arch, namely, to the enlarged ceratohyal and basihyal. 



The opercular series has remained fairly normal in external appearance. The rigid 

 preopercular braces the quadrate posteriorly. The interopercular shares the enlargement 

 of its associates, the cerato- and epihyal; it is fastened as usual by ligament to the small 

 angular bone of the mandible. The dermarticular is suddenly expanded downward, below 

 the quadrate-articular joint; it probably afforded insertion just in front of this expansion 

 to the transverse muscles of the mandible. The most remarkable feature of the entire 

 opercular apparatus is the reduction of its mobility. In the first place, the normal laterally- 

 expansive movement of the hyomandibular, which carries the opercular plates, appears to 

 be definitely stopped by a rigid transverse bar of bone connecting the hyomandibular with 

 the prootic and formed by both elements. Secondly, the hyomandibular articulation 

 with the sphenotic and pterotic is extended transversely as well as antero-posteriorly and 

 would appear to permit little if any motion. Thirdly, the opercular process of the hyo- 

 mandibular is directed downward and outward, while its cartilaginous tip extends inward 

 and is received into a deep funnel on the inner side of the opercular in such a way as to 

 allow at most only a small outward and inward swing of the posterior part of the opercular. 

 Again, the opercular has two contacts with the preopercular: one, a vertically extended, 

 bevelled facet just above the middle of the anterior edge of the opercular; the second, above 

 and in front of the first, comprising a short, forwardly-directed process with a concave 

 inferior contact just behind the prominent vertical ridge of the preopercular. This second 

 contact is shown on the right side only in the single specimen studied. The left opercular 

 lacks this process and may have been movable on its hyomandibular pedicle. Along with 

 all these indications of greatly restricted mobility a small but distinct fossa for the dilatator 

 operculi muscle remains beneath the dermal roof of the sphenotic and pterotic and even 

 in the dried skull it contains a number of tendinous bands which are apparently remnants 

 of this muscle; so it would not be surprising if in life the cartilaginous covering of the main 

 joint between the pedicle of the hyomandibular and the opercular permitted a minimal 

 movement of the opercular even in this exceptionally rigid skull. One may suspect that 

 the inhalation of water into the branchial chamber is effected chiefly by rhythmic movements 

 of the hyoid and branchial arches, since the enlarged hyoid arch retains its movable articula- 

 tion with the hyomandibular. 



Trichodon. — This is an isolated and puzzling form. The small skull (No. 285, Brit. 

 Mus. Nat. Hist.) shows but few definite indications of its relationships. In the dried 



