654 



Pr. HARKIS H. WILDER, 



I. External characteristics. 



The shape and proportions of the head, as well as the changes 

 during the growth of the animal, are best shown by the following 

 table, where the measurements are compared with those of other Per- 

 ennibranches and Derotremes. The accompanying figures show the 

 lines of measurement. 



From the table the following facts may be deduced. 1) The nasal 

 capsule of Siren and Siredon is proportionally much shorter than in 

 the others. 2) In the case of Siren this distance is less in the younger 

 specimen. 3) The heads of Siren and Siredon have the greatest pro- 

 portionate thickness dorso-ventrally. 4) The thickness is proportionately 

 greater in the younger Siren. 



The external surface of the head is, in young animals, quite 

 smooth, but in full-grown specimens, the subcutaneous parts cause 

 a number of external projections and grooves which may now be 

 studied. The large, rounded surface lying between the eye and the 

 external gills is caused by the excessive development of the subjacent 

 muscles, masseter and digastricus. The prominence extending dorsally 



