CLARK: THE ECHINODERMS OF PERU. 339 



while four of the remaining five are Panamic. The origin of the ophiu- 

 ran fauna is unquestionably Panamic; indeed, there is not a single 

 species which seems to have come into the Peruvian fauna from the 

 south, unless Amphiodia chilensis is a possible exception. 



In referring to the external features of a brittle-star it is customary to speak of 

 the body as the disc, and the rays as arms. On the sides of the arms, in parallel 

 vertical series, are borne the more or less conspicuous arm-spines. On the upper 

 surface of the disc lie the radial shields, a pair of plates at the base of each arm ; 

 they are sometimes covered by granules or small scales and so appear to be 

 wanting. At the center of the lower surface of the disc is the mouth, made up of 

 five radial slits meeting at a common center; between eacli pair of slits is a trian- 

 gular or wedge-shaped iiiterradial jaio, along the margins of which are the mouth- 

 papillae ; these papillae show great diversity in size, form, and position, and are of 

 particular importance in classification ; in Ophiothrix and some other genera they 

 are quite wanting. 



Key to the Brittle-Stars of the Peruvian Coast. 



Arms simple and unbranched. 



Disc closely covered with a fine granulation ; arm-spines short and 



appressed Opkioderma panamense 



Disc not granulated but covered witli more or less distinct scales ; arm- 

 spines not appressed. 

 Disc-scales smooth and free from spines and spinelets. 

 Lower surface of disc (between arms) covered with scales ; 3 mouth- 

 papillae on each side of jaw. 

 Outermost mouth-papilla very wide, equalling or exceeding the two 

 inner ones taken together ; size small, disc 2-3 mm. in diam- 

 eter ; arm-spines more or less pointed. 

 Disc-scales indistinct even when dry, appearing as though 

 covered by a thin skin; arm-spines wide and very flat; no 

 white spot at outer end of radial shield . . Amphipholis laevidisca 

 Disc-scales very distinct ; arm-spines slender and sharp ; radial 

 shields more or less white at outer end . . . Amphipholis pugetana 

 Outermost mouth-papilla not conspicuously wider than the others ; 

 size large, disc 5-12 mm. in diameter; arm-spines stout and 

 very blunt. 

 Radial shields large, their length more than one-third radius 

 of disc ; disc-scales coarse, fewer than 10 radial series in 

 each interradial area above and only 100-200 scales in 



each interbrachial space below Amphiodia grisea 



Radial shields small, their length about one-fourth of radius 

 of disc ; disc-scales small, about 15 radial series in each 

 interradial area and 500-1000 scales in each interbrachial 

 space below Amphiodia chilensis 



