1911] Michael: Chactognatha of the Sail Diego Region. 65 



Table 22 — Continued 



Appearance opaque opaque 



Collarette present present 



Tail in per cent of length 28-33 25-33 



Anterior fin, length and width .-.. 13-19 x 3.8 per cent 20 x 2.9 per cent 



Anterior fin, to posterior fin 6.3-8.7 per cent 6.8 per cent 



Posterior fin, length and width ... • 23-28 X 4.9 per cent 26 x 3.7 per cent 



Posterior fin, extends caudally .... to seminal vesicles to seminal vesicles 



Posterior fin, greatest width behind tail-septum behind tail-septum 



Posterior fin, per cent in front 



of tail-septum 36-40 35 



Seizing jaws 8-9 5-7 



Anterior teeth 4-5 5-10 



Posterior teeth 8-15 11-14 



In addition to the above characters, others not easily pre- 

 sented in tabular form are essentially the same in the two species. 

 The form of the body, position of greatest width, and the gradual 

 diminution on approaching either head or tail are very similar 

 in the two forms. Again, the form of the fins is nearly the same. 

 Finally, the extent of collarette and the position and form of 

 corona ciliata are greatly alike. 



A careful examination of the above table reveals but five 

 points of difference. They consist of width, width of anterior 

 fin, width of posterior fin, number of anterior teeth, and number 

 of seizing jaws. Let us then look at these differences a little more 

 closely. 



1. Width of Body. Owing to inshrinking of the lateral fields, 

 my measurement of 4.3 per cent is probably too small to represent 

 the width in fresh or living material. Granting, however, that 

 4.3 per cent is the true width, an examination of width measure- 

 ments of the San Diego species reveals greater variation than 

 from 4.3 to 5.7 per cent. S. enflata varies from 6 to 12 per cent, 

 <§. hexaptera from 7 to 11 per cent, <S'. lyra from 6.3 to 10.5 per 

 cent, and S. serratodentata from 3.4 to 6.5 per cent. Other 

 species show as great variation. It seems, therefore, that this 

 difference between 8. hispida and S. rohusta is of no specific 

 importance. 



