THE CLUPEOID CIL\NIUM AND THE SWIMBLADDER 453 



THE SKULL IN ITS RELATION WITH THE PRECOELOMIC DIVER- 

 TICULimi AND THE MEMBRANOUS LABYRINTH 



In this connection, the sides and base of the cranium are of 

 particular importance; description of other parts of the clupeoid 

 skull may be found in the papers of IMathews ('86) and Ridewood 

 C04). 



The clupeoid cranium, although highly specialized as to cer- 

 tain features, conforms in its more essential osteological char- 

 acteristics to the general structure of the skull of the lower 

 teleostean groups (pi. 1). The axis of the posterior part of the 

 skull is formed by the basioccipital, which articulates posteriorly 

 with the vertebral column, anteriorly with the medial plates of 

 the prootic of the two sides, and laterally with the exoccipital 

 bones of the two sides. Lateral to the exoccipital and the 

 prootic bones, and forming the most lateral part of the occipital 

 region of the cranium, is the pterotic bone. The anterior 

 medial cranial floor, formed chiefly by the two prootics, is 

 completed by the basisphenoid (figs. 7 and 8) with the hypo- 

 physial foramen intervening between that bone and the median 

 articulation of the medial plates of the two prootics. The ex- 

 treme lateral part of the cranium anteriorly is formed by the 

 sphenotic which articulates with the lateral edge of the prootic 

 bone. The posterior osseous capsule, containing the posterior 

 membranous vesicle of the swimbladder diverticulum, is con- 

 tained in the pterotic bone; the anterior osseous capsule con- 

 taining the anterior membranous vesicle forms a considerable 

 part of the mass of the prootic bone (fig. 5). 



These bones may now be described in detail. 



1. The basioccipital hone 



The condylar end of this bone is circular in form where it 

 articulates with "the first vertebra. The body of the bone is 

 hollowed on its lateral surface in such a manner that it consists 

 merely of a thin median plate separating two lateral concavities; 

 small lateral wings slightly deepen these concavities (figs. 4 and 



JOURNAL OF MORPHOLOGY, VOL. 33, NO. 2 



