MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 319 



Occipital Sclerotome in the Vertebrata. 



1. The centrum or basal part— the ' basi-occipital.' • 



2. Infero-lateral elements — the ' ex-occipitals.' 



3. Supero-lateral elements — the ' mastoids.' 



N.B. These elements coalesce very early with the petrosal in Mammalia generally ; 

 but in the smaller Cheiroptera, e. g. Vespertilio , Plecotus, and Molossus, they 

 coalesce with the ex-occipitals, and remain quite distinct from the petrosals. 

 In many, if not most Birds, the mastoids are not distinct from the ex-occipitals, 

 the outer margins of these bones having a ' periotic function.' In Chelonia 

 the mastoids are large and permanently distinct; but in the Crocodilia and 

 Ophidia they coalesce very early with the ex-occipitals ; in the Lacertilia they 

 are often distinct, but small ; whilst in the Batrachia they are not distinct from 

 the ex-occipitals. In Fishes the mastoids are often distinct, and are very large 

 in the Gadidse. 

 4 a. Supero-mesial elements — the inter-parietals. 



N.B. These are evidently often formed from a pair of lateral elements in the Mam- 

 malia. The single piece in the adult is largest in the ' lissencephalous ' mam- 

 mals, e. g. Mus, Talpa. In Birds they do not exist. In the Abranchiate 

 Reptilia the inter-parietal is single, and coalesces very early with the supero- 

 sub-mesial elements. They have no separate existence in ordinary Batrachia. 

 In Fishes the inter-parietal is single and generally remains distinct. 

 4 6. Supero-sub-mesial elements — the ' squama occipitalis,' or the ' epiotics.' 



N.B. These elements form the principal parts of the supra-occipital region in the 

 Mammals, and may be developed from a pair of lateral centres. In Birds 

 their inner margin is thin and squamous — the outer margin taking on the 

 ' epiotic ' function ; they coalesce very early, or are originally single in this 

 class. In the Abranchiate Reptilia they are small as compared with the 

 inter-parietal, and very early coalesce with it. In Fishes they are generally 

 distinct, and are filled with the upper part of the semicircular canals. 



Post-sphenoidal Sclerotome. 



1 a. The centrum or basal part — the ' basi-sphenoid.' 



N.B. This developes a very long exogenous ' rostrum ' in typical Birds, and a short 

 upper rostrum in Crocodilia and Chelonia. 

 1 6. Cortical inferior elements — the ' basi-temporals ' (nobis). 



N.B. These coalesce very early with each other and with the basi-sphenoid in 

 Birds. They probably exist in the embryos of the higher Reptilia ; for to us 

 it is evident that in struthious Birds, and in all the cold-blooded Ovipara, 

 the great ' rostrum ' is a cortical ossification. 

 2. The posterior infero-lateral elements — the ' petrosals.' 



2 y2 



