49] THE SKULL OF AM lURUS— KINDRED 49 



the dermosquamosal parts, and those derived from perichondria! ossification 

 the autosquamosal parts. His figures are clear and easy to understand. 

 The outer perichondria! lamella (for there are two, one on the outer perichon- 

 drium of the wall and one on the inner) is strongest where it is unprotected by 

 the dermosquamosal. In the younger stages the dermosquamosal was sepa- 

 rate from the autosquamosal, but gradually the parts become intimately fused. 

 He cannot say which of the tM^o ossifications appeared first. Toward the 

 posterior end of the roof of the lateral semicircular canal, the muscle fibres at 

 the insertion of the levator operculi ossify and fuse with the perichondria! 

 lamella. The articular surface for the hyomandibular ossifies later. 



In the 32 mm. Amiurus, the two elements described by Schleip for Salmo, 

 are present above the lateral semicircular canal. The dermal ossification con- 

 tains the lateral ;ine canal ossification and is partly fused with the perichondria! 

 lamella (Fig. 31). Toward the middle line of the cranium the inner end of the 

 dermal ossification is separated from the cartilage of the otic capsule by fibrous 

 connective tissue and connects by suture with another dermal ossification which 

 forms the margin of the median posterior fontanelle (Fig. 3). By fusion with 

 the otic perichondria! ossification (Fig. 31), the derma! ossification loses its 

 identity as a discrete ossification, as does the squam.osal element of man when 

 it fuses with the underlying periotic bones. Despite the fact that it is not 

 related to an otic ossicle, I think that this dermal ossification overlying the roof 

 of the lateral semicircular canal of Amiurus is the homologue of the mam- 

 malian squamosal. This homology however, does not apply to any other part 

 of the compound bone. The perichondria! ossifications are formed around a 

 center of ossification peculiar to fishes and called by Parker (1873), the pterotic 

 ossification. As in Salmo, the fibres at the dorsal end of the levator operculi 

 muscle have ossified and form a crest connected to the outer perichondria! 

 lamella and extending posteriorly above the hyomandibular. At the posterior 

 end of the roof of the lateral semicircular canal, the ventral arm of the post- 

 temporal ossification projects in beneath this crest (Fig 3). The dorsal arm 

 of the same ossification lies behind the crest of the epiotic ossification and above 

 the perichondria! ossification on the outer surface of the roof of the posterior 

 semicircular canal. 



Posterior to the hyomandibular articular surface on the dorso-latera! face 

 of the lateral semicircular canal wall, the opercular-mandibular lateral line 

 canal ends at this stage. The lateral line canal in the squamosa! sends down a 

 tubule which opens immediately above the dorsal end of the opercular- 

 mandibular canal and eventually unites with it as in the adult (Fig. 11). The 

 canal passes from the squamosal into the post-temporal, but before entering the 

 latter, the osseous canal enclosing the sensory canal, is independent (Fig. 3). 

 There is also a short interval between the end of this cana! ossification and the 

 post-temporal, where the canal lies unenclosed in connective tissue. 



