DIADEMATID^. 109 



non-crenulated. The biserial arrangement of the pores and the deep actinal 

 cuts may probably not be family characters ; that the extraordinary develop- 

 ment of the tube-feet in M. tiiherculata is no character of high order is proved 

 by the fact that in M. violacea these tube-feet are simple." If we examine 

 these characters with a little care we find that only the first one is remark- 

 able, for Doderlein (1906) has alread}^ noted the occasional presence of ophi- 

 cephalous and claviform pedicellarice, while the extremely minute serration 

 of the end of the triphyllous valves is surely not of family importance. The 

 structure of the stalk of the pedicellarioe, while very characteristic, differs 

 only in degree and not in kind from what we find in the other Diadematidae ; 

 even if essentially different, Dr. Mortensen could hardly consider it a family 

 character, for a similar difference in the Echinothuridae, he regards as a 

 characteristic of his genus Kamptosoma. Non-crenulate tubercles are not 

 unknown in the Diadematidae ; Dr. Mortensen gives them as one of the 

 characters of his genus Lissodiadema. The anchor-shaped spicules are, then, 

 the one characteristic feature of the Micropygidge ; we are inclined to add 

 also the remarkable biserial arrangement of the pores, though Dr. Mortensen 

 says that is " probably not" a family character. One feature of the internal 

 anatomy is very striking and seems to us w^orthy to be mentioned with these 

 two ; namely, the ends of the compass-rods of the " lantern " are perfectly 

 simple and similar, the outer end not being widened and more or less bifur- 

 cate as it is in other echini. Against these three notable peculiarities of 

 Micropyga, we have to set the important resemblances to the Diadematidie, 

 which are present. The arrangement of the alimentary canal and the struc- 

 ture of the " lantern " and teeth are essentially as in Echinothrix and Astro- 

 pyga. The form and structure of the test and the abactinal and anal systems 

 are distinctly Diadematoid. The composition of the compound plates of the 

 ambulacra is, in spite of the biserial arrangement of the pores, surprisingly 

 like Diadema, more so indeed than we find to be the case in Astropyga.^ 



^ Dr. Mortensen (1904, p. 42) makes a curious slip in regard to the value of the characters 

 shown by the ambulacra, foi he says : " 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 whicii 

 the adoral primary plate is a small plate, the following one being the largest, and the Echinoid type 

 in which the adoral 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." If this be true we are wrong in placing 

 Tetrapygus in the Arbaciada^, for Duncan and Sladen (1885) long since showed that its ambulacra 

 are '.' echinoid " and not " diadematoid " as are those of the Arbaciada;. Are we to presume that Dr. 

 Mortensen will establish a family " Tetrapygid;^ " under his " Tribus 4. Echinina," for this aberrant 

 genus ? 



