No. 2.] MORPHOLOGY OF THE MYXINOIDEI. 



199 



of the olfactory capsule by five processes, one median, two 

 lateral, and two ventral, as already described. The penulti- 

 mate and antepenultimate arches always appear concave for- 

 wards, when viewed from above. 



The extraordinary degree of variation in the external form 

 of Bdellostoma, and especially in the number of gills and teeth, 

 is a fact which has already been emphasized by Dr. Ayers. 

 It is even more remarkable to find a similar variation in the 

 more deeply lying structures, including the skeletal parts. 



a 



Some of the more important variations have already been 

 referred to, but in order to give a more adequate idea of 

 the actual amount of variation, we shall give as an example 

 the main points of difference found in the nasal arches of 

 a dozen specimens taken at random. 



The nasal arches are found to vary to a considerable extent 

 in number, form, and size, both relative and absolute. In 

 number, as has been stated, the nasal arches vary from eight 

 to ten, but they are usually nine in number. In this count 

 the two transverse bars of the olfactory capsule, though seri- 

 ally homologous with the nasal arches, are not included. The 



