DERMATODIADEMA. 101 



fathoms. These six species are so nearly related that they can be distin- 

 guished only when all their characters are taken into account. We find that 

 the number of coronal plates and of ambulacra! plates are important features, 

 while the relative size of the abactinal system, the number and arrangemert 

 of the anal plates, the distribution of miliary spines and tubercles, and even 

 the pedicellarise, variable as they are, afford characters which must not be 

 overlooked. Not having had a specimen of Tiiolle for comparison, we find it 

 difficult to make the distinction between it and glohulosum, tangible, for Doder- 

 lein's figures are too indistinct to enable us to determine whether apparent 

 differences in the ambulacra! tubercles and the actinostome are rea! or not. 

 We have been obliged, therefore, to be content with the rather marked dif- 

 ferences in the pedicellariae of these two species. 



Coronal plates numerous, 10-12 ; abactinal system small, about \ h. d. ; 



size large, 23-34 mm. h. d indicum. 



Coronal plates fewer, 6-9; abactinal system larger, usually about f h. d. 

 Anal system with 5-8 large plates around anus ; other anal plates 

 small and few, or wanting ; vertical diameter of test, .65-85 h. d. ; 

 ambulacra broad, usually exceeding .15 h. d. 



Pedicellariae valves long and slender (stout tridentate valves 

 about 1.5 mm. long, the width little more than half length ; 

 slender tridentate valves about 1.6 mm,, perfectly straight 

 and not widened at tip ; triphyllous valves about .50 mm.) globulosum. 

 Pedicellariae valves shorter and stouter (stout tridentate valves 

 about 1 mm. long, the width § of length ; slender tridentate 

 valves about 1.8 mm., expanded at tip and slightly curved; 



triphyllous valves about .33 mm.) molle. 



Anal system with more or less numerous plates, chiefly of small 

 size, those around anus seldom conspicuously bigger ; vertical 

 diameter of test usually under ,70 h. d., but may be .80 ; ambu- 

 lacra not so broad, usually under .15 h. d, 



Ainbulacral plates numerous (5-6 to each of largest interam- 

 bulacrals) ; buccal plates with spines ; valves of slender tri- 

 dentate pedicellariae slightly curved, meeting only near tip microtuberadatum. 

 Ambulacral plates fewer (3-4 to each of largest interambu- 

 lacrals) ; buccal plates commonly without spines, though 

 rarely in large specimens one or two may be present on 

 each plate ; valves of slender tridentate pedicellariae 

 straight, meeting for most of their length. 



Size large, up to 20 mm. h. d. ; ambulacra with 2-4 tu- 

 bercles on each plate, none conspicuously larger thau 



the others horridum. 



Size small, up to 13 mm. h. d., but rarely exceeding 10; 

 ambulacia with 1-3 tubercles on each plate, 1 tubercle 

 on every second or third plate noticeably larger than 

 the others antillarum. 



