730 I^r- J- BEARD, 



is a large animal, and its teeth are also of fair size. Probably they 

 are rapidly used up , and hence must be re-formed so rapidly that 

 one always finds three cones to each cusp. 



There are no points of much morphological interest in the teeth 

 of P. marinus, and except that the biting surface is horn, they 

 possess no points in common with the true teeth of Myxine and 

 Bdellostoma. 



The teeth of the Myxinoids. 



The differences obtaining between the teeth of Myxine and those 

 of Bdellostoma are so slight, that there would be no particular ad- 

 vantage in giving full and separate descriptions of each. The most 

 striking difference is the much greater size of those of Bdellostoma; 

 the latter are also better developed histologically. While the median 

 tooth of Bdellostoma has a length of from five to six millimetres, 

 that of Myxine only measures one or two millimetre. The same pro- 

 portion also holds for the lingual teeth of the two genera. 



The arrangement of the teeth in the mouth has been de- 

 scribed by Johannes Müller (15, pp. 78, 79, more fully p. 120). It 

 need hardly be remarked that his account is perfectly correct. 



In both forms there is a large median tooth on the gum, not 

 very far within the oral cavity, and there are two rows of teeth on 

 the tongue, the 1 i n g u a 1 t e e t h. In Myxine there are eight teeth in 

 the first lingual row, and eight or nine in the second. Bdellostoma 

 has more : either eleven {B. liexatrema), or twelve {B. Jieterotrema) ^ ) 

 (15, p. 79). This shows that the forms with the most gill clefts have 

 also most teeth, and, a priori, points out that we have a gradual de- 

 generation in the teeth from B. heterotrema to the Petromyzon- 

 tidae. It will be worth while to quote in full what Johannes Müller 

 has to say about the teeth , more especially as his account is the 

 fullest we possess: and, so far as it goes, it is exceedingly good. 



He devotes an entire „Capitel" to these structures and writes as 

 follows (15, p. 120). 



„Die Zähne der Bdellostomen und Myxinen gleichen im Allge- 

 meinen denen der Petromyzen, sie sind hornartig, hohl, nur sind sie 



1) According to Dr. Günther there is but one species of Bdello- 

 stoma. The species distinguished by Müller are merely varieties. 



