42 



ECHINOIDEA. I. 



basal part is an uiidivided cavity in accordance with the faet that muscles are only running in one 

 direction between the two valves. (In the coinmon three-valved pedicellariæ muscles, as is well known, 

 run in two diverging directions from each valve, and the apophysis may be taken to serve chiefly for 

 the attaching of these muscles). The structure of the stalk is as in D. papillata. Other kinds of pedi- 

 cellariæ do not appear to be found in this species (genus). 



The spines have no < hair -covering on the outer layer, as was the case in the three preceding 

 species; but the outer layer itself is beautifully and regularly striped longitudinally, and is in trans- 

 verse sections seen to be divided into areas, one area for each raised ridge. The more conspicuous 

 ridges are formed by two parts of equal height, joined almost to the point (PL XI. Fig. 3). 



< Ingolf », st. 73 (62° 58' N. Lat. 23° 28' W. L. 486 fathoms. Bottom temperature 5° i). 3 specimens. 



Hitherto the species was only known from the Faroe Channel, from 530 — 542 fathoms. 



The smallest of the specimens in hånd (diam. 10'"'", height 7""", longest spine 27""") agrees 

 exactly with the description of Porocidaris gracilis Sladen (op. cit.). The form is the same; the radioles 

 are not separated, only one tubercle on each ambulacral plate, no openings in the genital piates — as 

 in P. gracilis \ onlj- the colour is more light (bleached) than in S lad en' s specimen. There can be no 

 doubt, however, that it is a young P. pitrpurafa , and P. gracilis Sladen must then, as supposed by 

 Bell (op. cit. p. 142) be taken to be synonymous with P. purpurata. 



It is especially by the spines that the young P. purpurata differs from the grown one. In 

 Sladen' s specimen they were finely striated longitudinally, the ridges being very slightly prominent 

 and marked with very faint and indistinct serrations . In the specimen in hånd, which is a smaller 

 one, the spines are very different between themselves, some are provided with rather highly serrate 

 longitudinal ribs, others are densely covered with coarse thorns, without any trace of longitudinal ribs; 

 a couple are only faintly serrated, and a single one of the uppermost ones is completely smooth, quite 

 as in the grown P. purpurata. Also in the grown one the lower radioles are rather distinctly serrated, 

 while the upper ones, with the exception of a few coarse thorns, only are finely striated longitudi- 

 nally. The radioles round the mouth are serrated as in the grown one, only, however, with i — 2 teeth 

 on either side. 



vSladen's .specimen was taken S. W. of Ireland on 51° i' N. Lat., ii°5o'W. L., 750 fathoms. 



Table of the Cidarids oceurring in the norlhern Atlantic and the Mediterranean. 



1. Pedicellariæ 2-valved; the spines with highly developed neck .... Porocidaris purpurata W. Th. 



— 3-valved ; the spines with short neck 2. 



2. The globiferous pedicellariæ, as well the large as the small ones, 

 with an unpaired tooth at the point of the blade; the mouth does 

 not reach to the point of the blade, and is most frequently irre- 



gularly limited below. The spicules formed as spinous arcs Dorocida ris papillata (Leske). 



The large globiferous pedicellariæ withouth end-tooth; the 

 mouth reaches to the point of the blade, and is regularly limited 

 below. (Sometimes au unpaired end-tooth may apparently be 



