1897.] on the gbinding-teeth of the manatee. 595 



Eeference Letters. 



as. Angulosplenial bone. 

 bh. Basihyal. 

 cbr-. Second ceratobranchial. 

 ch. Ceratohyal. 

 d. Dentary bone. 



fl. Poramen laterale in figs. 9 and 10 ; and in figs. 7 and 8 the 

 sinus which later develops into the foramen. 

 ffhe. M. geniohyoideus esternus. 

 ffhi. M. geniohyoideus internus. 

 h, k', h". Parts of the anterior or hyoidean cornu. 



ha. Convex surface by which the ceratohyal articulates with the 



palatoquadrate cartilage. 

 hhr. Hypobranchial plate. 

 h(/. M. hyoglossus. 

 hffs. Hyoglossal sinus. 

 m. Internal or mesial part of the mandibular cartilage of the larva. 

 m,'. Lateral part of the mandibular cartilage of the larva. 

 mm. Mentomeckelian bone. 

 ok. M. omohyoideus. 

 P^t P^ 'P^jP*- The four divisions of the M. petrohyoideus. 

 pa. Processus anterior. 

 pal. Processus antero-lateralis. 

 ppl. Processus postero-lateralis. 



s. Space enclosed between the ceratohyal, basihyal, and first cerato- 

 branchial. 

 s'. Space enclosed between the hypobranchial plate and the proximal 



ends of the first and second ceratobranchials. 

 sh. M. sternohyoideus ventralis. 

 sh'. M. sternohyoideus dorsalis. 

 sp^, sp^. Cartilaginous spicula of the first and third branchial arches. 

 t. Thyrohyal. 

 tf. Thyroid foramen. 

 ■V. Ventral splint-bone. 

 IX. Aperture in membrane through which the glossopharyngeal 

 nerve passes. 



5. On the Number of Grinding-Teeth possessed by the 

 Manatee. By Oldfield Thomas^ F.Z.S., and R. 

 Lydekker, F.R.S., F.Z.S. 



[Received March 16, 1897.] 

 (Plate XXXVI.) 



With one exception, the authors who have treated of the 

 dentition of the Sirenians, from Cuvier in 1817 to the most recent 

 writer on the subject, have estimated the number of cheek-teeth on 

 each side of each jaw in this genus at from 6 to 8 to about 11 or 12. 



The single exception was Dr. Krauss, of Stuttgart, who, in one 

 of his several papers on Sirenian skulls, writing solely of the 

 method of growth of the jaw-bone, and evidently without any 

 idea of the importance of the remark in regard to the number of 

 teeth, says' : — 



" AUein, vergleicht man den letzten Backenzahn eines jungen 



1 Arch, Anat. Phys. 1862, p. 422. 



