178 Scientific Proceedings, Royal Dublin Society. 



mandibular, and also partially overlaps the triangular intercalary 

 cartilage. The upper end of the bone articulates with the posterior 

 process of the hyomandibular. The preoperculum is connected 

 behind and below to the other three bones of tlie gill-cover. 



The interoperculum (PL Y., fig. 11) is a long, thin bone, con- 

 nected above with the preoperculum, behind with the suboper- 

 culura, and in front with the mandible. Near the posterior end a 

 strong spur of bone projects from the internal surface, in front 

 of which a concave articular surface is formed, which supports the 

 articulation of the stylohyal with the epihyal. 



The subopercnlum is a curved scale of bone, lying below the 

 operculum proper, which overlaps it. It is also overlapped by the 

 preoperculum and intei'operculum, and is connected to these three 

 bones by ligamentous fibres. 



The operculum proper is irregularly triangular. The superior 

 angle presents, on its internal surface, an articular cavity for the 

 end of the posterior process of the hyomandibular. The upper 

 border is nearly straight ; the anterior, convex ; and the inferior, 

 concave. . Internally, a groove runs from the articular cavity to 

 the posterior angle for the insertion of a muscle, which adducts 

 the bone (see PI. Y., fig. 12). 



The ligaments which have been mentioned above may be re- 

 capitulated as follows : — Palato-premaxillary (see PL YII., fig. 

 13), palato-vomerine, ento-pterygoid-prefrontal, maxillo-ethmoidal : 

 these are all very strong. Distinct ligaments also connect the max- 

 illa with the premaxilla, and both these bones with the pre-ethmoid 

 cartilage ; ligamentous fibres connect the lachrymals and nasals to 

 all the surrounding bones ; in particular, a strong network of fibres 

 binds the two premaxillas together, and passing upwards, covers 

 the anterior ends of the lachrymals and nasals. The pre-ethmoid 

 cartilage is entirely hyaline: so is the strong rod of persistent 

 cartilage,^ which lies between the ethmoid, vomer, prefrontal, and 

 parasphenoid ; the disks of fibro-cartilage between the maxilla and 

 ethmoid, and between the maxilla and premaxilla, are nearly all 

 fibrous- tissue on the periphery, and nearly pure hyaline cartilage 

 in the centre. Covering the lachrymal where it articulates with 

 the prefrontal is a substance looking to the naked ej^e very like 



1 The unossified portion of the cranial trabeculse. 



