70 



CONNECTICUT GEOL. AND NAT. HIST. SURVEY. 



[Bull. 



while in other species they are more or less completely covered 

 by the more superficial scales of the disk (Plates XIII and XIV). 



Fig. 9. Oral surface of disk and bases of arms of 

 Ophiopholis aculeata; a, adoral plate; d, disk, with 

 small, blunt spines covering the plates; e, tentacle 

 scale; g, genital slit; m, madreporic plate (modified 

 oral shield) ; 0, oral shield ; p, oral papillae ; s, arm 

 spine; t, tooth; u, under arm plate. 



Arm plates. — The arms are covered on all sides by a com- 

 plicated system of thin plates and spines. The form, number, 

 and arrangement of these parts are used in the classification of 

 the group, so that one must examine them closely in order to 

 determine the name of any one of our native species. A single 

 row of plates, the upper arm plates, usually lies along the aboral 

 surface of each arm; and a similar row, the under arm plates, 

 often with plates of different size, occupies a similar position on 

 the oral surface (Fig. 9). Beside these upper and under arm 

 plates, and occupying more or less of the lateral surfaces of the 

 arm, often occur one or more series of smaller plates, and these 



