MR. J. W. DAVIS ON" CHONDROSTETJS ACIPENSEROIDES. 615 



much more nearly to the existing types. In this specimen the 

 length of the body from the base of the pectoral fin to the pedicle 

 of the caudal fin is 24 inches, the length of the caudal is 12 inches : 

 in Egerton's specimens the proportions are as 15 inches to 9 inches ; 

 though it is possible that as the caudal fin was not attached to the 

 body in the latter instance, the estimation of the proportionate 

 lengths of the body and tail may have been exaggerated in the case 

 of the latter. The ventral and dorsal fins are described as being 

 midway between the base of the pectoral and that of the caudal ; 

 in this specimen the distance between the pectoral and ventral is 

 14 inches, and between the latter and the caudal 10 inches. The 

 dorsal fin of this specimen is readily distinguished by the large 

 number of short rudimentary or accessory rays forming its anterior 

 portion ; and the pectoral fin is possessed of a strong anterior 

 spinous ray very similar to that of some recent Sturgeons. 



Some differences in the cranial anatomy have been already 

 mentioned. The division of the scapular arch into three parts — 

 the suprascapula, the scapula, and the coracoid — in this specimen 

 appears to be undoubted, whilst in those previously described the 

 scapula and coracoid are said to be united. In the existing 

 Sturgeons the two latter ossifications of the shoulder-girdle are 

 separate*, and in the Ganoid fishes generally this is also the case. 



The mandibular suspensorium, in the examples described by 

 Sir P. Egerton, is stated to consist of a large epitympanic bone 

 connected with the tympanic pedicle ; the succeeding cartilages, 

 the mesotympanic and hypotympanic, which are represented in the 

 recent Sturgeon by two cartilages, are united to form one bony plate. 

 This united bone is very small and feeble as compared with the 

 epitympanic, and is considered ill-adapted, either in form or dimen- 

 sions, to afford an articulation strong enough to support the 

 mandibular and maxillary organs. Dr. Giintherf states that 

 the suspensorium in the Sturgeon is movably attached to the side 

 of the skull, and consists of two pieces, a hyomandibular (epitym- 

 panic) and a symplectic, which now appears for the first time as a 

 separate piece, and to which the hyoid is attached ; as regards the 

 second part of the suspensorial apparatus the Sturgeons are distinctly 

 in advance of the palasoniscoid Ganoids, in which Dr. Traquair has 

 seen no trace of symplectic t. 



A careful comparison of the numerous specimens of the genus 

 CJiondrosteus, at present located in the New Natural History 

 department of the British Museum, has led to the conclusion that 

 there is no specific difference between the species Chonclrosteas 

 acipenseroides, Agassiz, and C. crassior, Egerton. The characters 

 separating the two, as stated by Sir Philip Egerton, are slight, con- 

 sisting in the relative thickness of the cranial bones and the position 

 of the anal fin. The former difference can scarcely be maintained, 

 because the specimens merge gradually the one into the other, and 



* Prof. W. K Parker, " On the Shoulder-girdle and Sternum." 

 t ' Study of Fishes/ 1880, p. 76. 



| Palaeoniscidse (Palagontographical Soc, toI. xxxi, 1877, p. 17) 



