43 



I must here again affirm that characters of primary importance for classification 

 are often found in structures of evidently very little physiological importance (spi- 

 cules f. i.); I even think that we must esj)ecially look out for systematic characters 

 among the features likely to be of little or no physiological importance, such struc- 

 tures being little liable to be altered for any special use. For the rest it is rather 

 rash to declare without reservation — and without proofs by physiological experi- 

 ments — that the physiological importance for instance of the superficial structure 

 of the tubercles is exceedingly small, and on the other hand that the number of 

 interradial coronal plates in Cidarids for instance is of physiological importance. 



The structure of the ambulacra, on which Duncan lays so great stress, is 

 certainly of the highest systematic importance, and it is Duncan's great merit to 

 have shown by his careful studies on the anatomy of the ambulacra of the Echinids 

 that different types of compound plates exist'). He establishes six types, viz. the 

 cidaroid, the diadematoid, the arbacioid, the cyphosomatoid, the diplopodous and 

 the echinoid types. Of these the arbacioid, diplopodous and cyphosomatoid struc- 

 tures are only modifications of the diadematoid; I can only admit three different 

 types, viz. the cidaroid type, with simple primaries which do not combine to form 

 compound plates, the diadematoid type in which the adorai primary plate is a small 

 plate, the following one being the largest, and the echinoid type, in which the 

 adorai component is the largest, and never a demi-plate, the following being smaller. 

 But these features do not present generic or family-characters; they are of higher value. 

 All the families of Ectobranchiata may be arranged in three groups: namely with 

 simple^), or diadematoid or echinoid ambulacra; these are then characters of orders. 

 The minor variations in the ambulacral structure may present generic characters, 

 but scarcely any of higher value. Thus Duncan, when using exclusively the minor 

 variations in the ambulacral structure for subdividing the family Diadematidœ, gets 

 such surprising results as to make Centrostepbanus aud Hemipedina subgenera of 

 Diadema and to place Echinothrix and Astropyga (to which Cluetodiadema and Lisso- 

 diadema should probably have been added) in another subfamily. After the dia- 

 gnoses given by Duncan of the two subfamilies Diadematince and Pedininœ, one might 

 as well transfer all the genera of the Diadematince to the Pedininœ and vice versa. 



We must then seek other characters for grouping the genera, and there we 

 meet at once the „very popular and useful" structures, the crenulation and perfora- 

 tion of the tubercles. In fact Duncan has not given any reason for neglecting the 

 characters from these structures, except the assertion that „it may be stated, as a 

 general truth, that if these genera (viz. the group of the fossil genera which is 

 characterized by having numerous small tubercles placed actinally and at the 



■) On the Anatomy of the Ambulacra of the recent Diadematidæ. J. Linn. Soc. Zool. XIX. 1885. 

 On the Structure of the Ambulacra of some Kossil Genera and species of Regular Echinoidea. Quart, 

 .lourn. Geol. Soc. XLI. 188.5. 



') The simple ambulacra do not make a very distinct type, all ambulacra being originally simple. 



