172 MICROSCOPY FOR BEGINNERS. 



each, with one anterior and one posterior central 

 spine, octondrim, 8. 



e. Spines in two transverse rows, not projecting be- 

 yond the ends of the caudal branches, spino- 

 sulus, 9. 



e. Spines in two transverse, highly - arching rows, 



the posterior longest and projecting beyond 

 the ends of the caudal branches, longispinosus, 

 10. 



f. Back with a subcentral, transverse hedge of large 

 spines, scales double, acanthodes, 11. 



f. Back without a distinct spinous hedge, scales not 



double, spinifer, 12. 



g. Spines in four transverse rows, five spines in each, 



acanthophorus, 13. 



g. Spines in transverse rows, less than five spines in 

 each, enormis, 14. 



1. CHITON OTUS PODURA. 



Clicetonotus, or bristle-back, is rather a misnomer for 

 a species with a perfectly smooth dorsum, yet such a 

 one is not uncommon. The spines and other dorsal ap- 

 pendages are here represented by two hairs standing al- 

 most vertically on the neck, and two on the rear part of 

 the back. These are usually seen with difficulty, but 

 they are present on all the species, even the scaly 

 and the spinous ones. The egg of this species is 

 also smooth. Ehrenberg called this Icthydium po- 

 dura. 



