22 CYCLOSTOMES. 



Petromyzon marinus, the innermost teeth of four of the lateral series on 

 each side are bicuspid, or consist each of two cones, which are 

 confluent at the base. There are twenty converging rows of labial 

 teeth in this species, and from four to eight teeth in each row. 



The single tooth supported by the palatal cartilage (1) and analo- 

 gous to that in the myxinoids, consists here of two horny cones, 

 placed in the transverse direction, and joined, in the Petr. marinus, 

 in the median line. In the lampern {Petr. fiuviatilis) the cones are 

 more remote. The matrix of this tooth is hollow at the base, and 

 is supported on a conical process of the palatal cartilage, which 

 Cuvier describes as the upper jaw. The broad bicuspid palatal tooth 

 is opposed by the dentated semilunar horny plate, with which the 

 cartilage, representing the lower jaw, (2) is sheathed. This plate 

 consists of eight conical teeth, laterally united together ; its repro- 

 ductive matrix is fixed upon a prominent semilunar ridge at the 

 anterior part of the mandibular cartilage. 



The lingual teeth consist of three dentated horny plates, the 

 dentations being much smaller than in the palatal or mandibular 

 plates, or than in the lingual plates of the myxinoids. But their 

 analogy with the latter can be readily traced, the third or posterior 

 lingual plate of the lampreys evidently corresponding with the two 

 posterior lingual plates of the myxinoids conjoined. 



Each of the anterior lingual plates is slightly concave, with the 

 mesial extremity abruptly bent towards the upper surface of the 

 mouth ; its anterior margin is divided into eleven sharp-pointed, 

 recurved, minute, dental processes. The posterior and inferior dental 

 plate may be said to consist of two similar but smaller semilunar 

 pieces, with the mesial margins approximated and conjoined ; the 

 number of dentations on each of the lateral moieties of this lingual 

 armature, is seven. 



The mode of development and reproduction of all these teeth 

 is the same both in the myxinoids and lampreys. The matrix is 

 persistent, as in most other horny productions, and the new conical 

 tooth is developed immediately within and beneath the base of the 

 old one ; the same secreting surface which formed the one, produces 



(1) PI. 2, tig. 5, G. (2) ih. b. 



