Figure 280.— Dtodon holocanthus (center), 



with Chilomycterus schoepfi (above) 



for comparison : lower left, nasal 



region as seen externally and scales 



from upper middle region of body of 



C. schoepfi, and, lower right, nasal 



region as seen externally and scales 



from upper middle region of body of 



D. holocanthus; dotted line on Figure 



of C. schoepfi shows the course of the 



lateral line canals and their major 



pores, as deciphered by placing drops of 



ink on each pore found by microscopic 



search of a partially drying specimen. 



Otic Region. 



Pterotic. —Expanded posterolaterally into a 

 flattened, almost vertical, supporting strut for the pec- 

 toral girdle; broadly cartilage filled along its medial 

 edges; articulates by interdigitation anterodorsally with 

 the sphenotic, anteroventrally with the prootic, postero- 

 ventrally with the exoccipital, and dorsomedially with 

 the epiotic. At a concavity on the posterior surface of its 

 posterolaterally expanded portion, the pterotic articu- 

 lates by fibrous tissue with the anterior end of the supra- 

 cleithrum. Along the anterolateral edge of its ventral sur- 

 face the pterotic supports the hyomandibular. This 

 articulation between the pterotic and hyomandibular is 

 by fibrous tissue anteriorly, but posteriorly the two bones 

 become extensively interdigitated with one another 



Sphenotic. —Confined to the dorsal and lateral 

 portions of the skull and not entering into the formation 

 of the wall of the orbit; cartilage filled along all. of its 

 edges of articulation with the other cranial bones; ar- 

 ticulates by interdigitation anteriorly with the frontal, 

 medially with the supraoccipital, posteromedially with 

 the epiotic, posterolaterally with the pterotic, and ven- 

 trolaterally with the prootic. The region of articulation 

 between the sphenotic and prootic forms a concave sur- 

 face to which the anterior half of the dorsal edge of the 

 hyomandibular is held by fibrous tissue. The dorso- 

 lateral edge of the sphenotic possesses a stout, pronglike, 

 lateral process which serves as the place of origin for 

 muscles which insert on the operculum. 



Epiotic— A thin, more or less squarish plate; 

 cartilage filled along all of its edges of articulation with 

 the other cranial bones; articulates by interdigitation 

 anteriorly with the sphenotic, medially with the supra- 

 occipital, posteriorly with the exoccipital, and laterally 

 with the pterotic. 



ventromedially with the basioccipital. Posteromedially 

 the dorsal edges of the exoccipitals interdigitate with one 

 another so that the dorsal as well as the lateral walls of 

 the foramen magnum are formed by the exoccipitals. 

 From the posterior end of its ventrolateral edge the exoc- 

 cipital possesses a pair of short but sturdy processes 

 which overlie and articulate by fibrous tissue with the 

 anterolateral region of the first vertebra, these processes 

 being the modified exoccipital condyle. 



Supraoccipital. —A flat plate anteriorly, but drawn 

 out posteriorly into a much depressed spine which 

 reaches to about the level of the end of the first vertebral 

 centrum; articulates by interdigitation anteriorly with 

 the frontals, anterolaterally with the sphenotics, and 

 posterolaterally with the epiotics and exoccipitals. The 

 ventral surface of the anterior half of the supraoccipital 

 spine interdigitates with the dorsomedial surfaces of the 

 exoccipitals. 



Prootic. — Cartilage filled along all of its edges of 

 articulation with the other cranial bones, except an- 

 teriorly; articulates by interdigitation anterodorsally 

 with the frontal, anteromedially with the ptero- 

 sphenoid, anteroventrally with the parasphenoid, 

 posteromedially with the basioccipital, posteriorly with 

 the exoccipital, posterolaterally with the pterotic, and 

 laterally with the sphenotic. Medially on its ventral sur- 

 face the prootic articulates with the lateral edge of the 

 posterior end of the parasphenoid. This articulation is 

 usually through cartilage, and only in large specimens do 

 the two bones become interdigitated to any appreciable 

 extent. Cartilage persists in this region long after it has 

 at least mostly disappeared from external view between 

 the region of junction of the basioccipital, exoccipital, 

 and prootic and the region of junction of the prootic, 

 exoccipital, and pterotic. There is no evidence of a 

 myodome. Along the concave area at the region of ar- 

 ticulation between the prootic and sphenotic, the 

 hyomandibular is supported through fibrous tissue. This 

 support is strengthened, however, by the extensive inter- 



