MUSEUM OF COMPARATIVE ZOOLOGY. 247 



from Madeira. As to arrangement, the thick stumps and thorny grains, of 

 which the most numerous are 32, 33, are concentrated in the centre of the 

 disk, while the coarse spines are sparsely distributed from the centre awards 

 the periphery. 



Next comes 0. lusitanica, the commonest species, or variety, of some parts 

 of the coasts of France and Portugal, and even extending into the Mediter- 

 ranean. Somewhat larger than 0. echinata, it is at once distinguished by 

 the greater absolute size and thickness of the disk-stumps, and by the more 

 numerous thorns at their ends ; for, whereas 0. echinata commonly has stumps 

 simply forked, and never with more than three terminal thorns, 0. lusitanica 

 has them with a crown of four, five, and even six thorns (PI. II., Figs. 15, 16, 

 22). A young one whose disk was 5.5 mm. in diameter, had clavate or trilid 

 stumps, and some with four tei*minal thorns (Figs. 16-18); a still smaller 

 one, with a disk of 5 mm., had clavate stumps (Fig. 23) also on the radial 

 shields, where they are scarcely ever seen in the adult. On a specimen from 

 Naples, disk 8 mm., the stumps were chiefly stout cylinders with a crown of 

 five thorns (Fig. 15) ; but there were, besides, a few stout spines, some colum- 

 nar with two side thorns and six terminal (Fig. 19), others with a swelled 

 cluster of numerous thorns at the tip (Fig. 20). It should here be added that 

 spines are so rare in this species as almost to be accidents ; while the stumps 

 are of so even a height and so closely set as to give to the upper disk the 

 figure of a regular five-rayed star, in which the radial shields are sharply 

 defined. Of many examined, one from St. Va-est la Ilouge, France, had an 

 occasional spine of a type like that found in O. echinata and O.frayilis, but 

 much thicker (Fig. 29 : compare Figs. 1,4 2). To continue among rare forms, 

 an individual from the lies Chauses had small toothed grains and others 

 more elongated, derived from them (Figs. 25, 26) ; and 26 may be still further 

 elongated by the shooting up of the three central thorns, and thus take on 

 the character of a small spine ; this was found in one specimen from the lies 

 Chauses (Fig. 27). Among stumps, Figs. 21, 24, are rare eccentric shapes 

 derived from 15. Fig. 28 is an elongated clavate stump, comparable 

 to 9 in 0. echinata, 44 in 0. fragilis, and 53 in O. alopecurus, and like 

 them found near the edge of the disk. Of eighteen specimens examined 

 with special care, viz. two from Naples, one Portugal, eight St. Va-est la 

 Ilouge, seven lies Chauses (France), ranging from 5 to 13 mm. in diameter 

 of the disk, there were five specimens in which the. disk-stumps had not 

 developed beyond the two or three forked forms, but, as before mentioned, 

 larger and stouter than in 0. echinata of the same size-. The other thirteen 

 had more or less cylindrical stumps with crowns of four to seven thorns, and 

 two of them had also thorny grains (Figs. 25, 26). Among five adult speci- 

 mens, the average of the arm to the disk was as 5 : 1. The upper arm- 

 plates are decidedly angular ; the arm-spines about as in 0. echinata,, little 

 variable, and with thirteen to seventeen thorns on an ed-e. 



