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Profs. T. W. Bridge and A. C. Haddon. [June 20, 



membrane, in which the claustrum and the ascending process of the 

 scaphium are imbedded. 



Of the four Weberian ossicles the claustrum has no physiological 

 relations to the atrial cavities {atria sinus imparls of Weber), but 

 merely strengthens the wall of the neural canal behind the exoccipital. 

 Each scaphium has a spatulate process which fits into and completely 

 closes the corresponding external atrial aperture, and at the same time 

 forms the outer wall of the atrial cavity of its side, and also a rounded 

 condylar process for articulation with the centrum of the first 

 vertebra. The intercalarium is usually represented by an elongated 

 or discoidal nodule imbedded in the stout ligament (" interossicular 

 ligament") connecting the scaphium with the tripus, and even if 

 horizontal and ascending processes are present, the ossicle never 

 articulates with the centrum of the second vertebra to which, as a 

 modified neural arch, it belongs. The tripus is always a tripartite 

 ossicle with its posterior or crescentic process imbedded in the dorsal 

 wall of the air-bladder ; the anterior process is directed forwards 

 parallel to the long axes of the complex and first centra, and opposite 

 the external atrial aperture of its side is connected by the trans- 

 versely- disposed interossicular ligament with the convex outer surface 

 of the spatulate process of the scaphium. The articular process 

 usually articulates with the lateral surface of the vertsbral centrum 

 (the third), of which it is a modified transverse process ; very rarely 

 (e.g., Auchenipterus) is the process directly continuous with the 

 neural arch. 



The Weberian ossicles, or at all events the free portion of the 

 tripus and the intercalarium, are enclosed within a membranous 

 saccus paravertebrals, the anterior wall of which is perforated by the 

 interossicular ligament as the latter passes forwards from the tripus 

 to its attachment to the scaphium. Unlike the Cyprinidse, the com- 

 plete closure of the external atrial aperture by the spatulate process 

 of the scaphium and the minute size of the hypoglossal foramen in 

 the Siluridse completely cut off all communication between the cavity 

 of the saccus and the cranial cavity. 



The first spinal or hypoglossal nerve perforates the exoccipital. 

 The second and third spinal nerves emerge from the neural canal 

 between the claustrum anteriorly and the arch of the complex 

 vertebra behind, but are invariably separated by the ascending 

 process of the intercalarium whenever that process is developed, as 

 in Macrones, Liocassis, and Bagroides. The fourth and fifth spinal 

 nerves traverse the neural arch of the complex vertebra, and the sixth, 

 the arch of the fifth vertebra. The additional spinal nerve described 

 by Sagemehl in Silurus glanis as emerging between the claustrum and 

 the ascending process of the scaphium, we have never met with, 

 although our attention has been specially directed to that point. 



