58 THE VERTEBRATE SKELETON. 



In Lampreys the lingual apparatus (fig. 4, C, 9) is well de- 

 veloped, but not excessively so. It consists of a long median 

 cartilaginous bar which ends in front with a semicircular piece 

 of cartilage supporting the median part of the tongue. 



In both Myxinoids and Lampreys there is a complicated 

 branchial basket apparatus, but while in Myxinoids the basket 

 apparatus is interbranchial, formed deep within the head 

 near the hypoblastic lining of the throat, in Lampreys it is 

 extra -branchial and formed outside the head cavities (fig. 

 4, 10). The two sides of the basket apparatus in Myxine are 

 not symmetrical. In the interbranchial basket apparatus of 

 Myxinoids the hyoid and first and second branchial arches 

 can be recognised. Traces of the interbranchial skeleton of 

 Myxinoids can be detected in Lampreys, and similarly in 

 Myxinoids, there are indications of the extrabranchial skeleton 

 of Petromyzon. The branchial basket in Lampreys forms at 

 its posterior end a kind of cup which supports the pericardium 

 (fig. 4, 11). 



A remarkable Cyclostome named Palaeosj)ondylus l has re- 

 cently been described from the Scottish Old Red Sandstone. 

 It differs however from all living Cyclostomes, in having a 

 spinal column formed of distinct vertebrae with well-developed 

 neural arches. The caudal fin is well developed and the dorsal 

 radialia are forked as in lampreys. The skull is well calcified 

 and the auditory capsules are specially large. The mouth is 

 very similar to that of lampreys, being circular and without 

 jaws ; it is provided with barbels or cirri. There is no trace 

 of limbs and the average length is only about 1 1J inches. 



1 R. H. Traquair, Ann. Nat. Hist. vol. vi. 1890, p. 485 ; P. Phys. Soc. 

 Edinb. vol. xn. 189293, pp. 8794, and 312320. A. Smith Woodward, 

 Nat. Sci. vol. in. p. 128, 1893. 



