CLYPEASTRIDAE. 19 



CLYPEASTRIDAE Agassiz. 



This family includes the largest clypeastroids known and very few of the 

 species are less than 75 mm. in length when fully grown. Although the arrange- 

 ment of the auricles and the lantern-muscles (see Jackson, 1912, Mem. Boston 

 Soc. Nat. Hist., 7, p. 196), the structure of the spines, and the number, variety 

 and form of the pedicellariae all indicate that the family as a whole is the least 

 specialized in the suborder, yet many of the species show by the completely 

 formed petals, the very flat test, and the position of the anus a very considerable 

 specialization. While the test is decidedly flattened in most of the species, 

 there are some in which it is more or less highly arched and the generic position 

 of these has been a source of considerable discussion. Among these, an Austra- 

 lian species, described in 1878 by Tenison- Woods under the name Echinanthus 

 tumidus, is particularly interesting and there is no doubt Bell was quite right 

 in making it the type of a new genus, Anomolanthus. Another of the high 

 forms and one of the best known members of the family has long borne the name 

 of Echinanthus rosaceus but under the International Code, the name Echinan- 

 thus may not be used for a clypeastroid (see H. L. Clark, 1911, Ann. Mag. Nat. 

 Hist., ser. 8, 7, p. 594). Moreover rosaceus is undoubtedly the type of Lamarck's 

 genus Clypeaster and consequently, if the highly arched and the flattened species 

 of Clypeastridae are to be separated generically from each other, it is the former 

 and not the latter which retain the name Clypeaster. But after a careful study 

 of nearly all the recent species, I have concluded that the gradations in the 

 form and structure of the test are so complete, it is better to let all the species 

 of the family, recent, Quarternary, and Tertiary, except the Australian form 

 referred to above as Anomolanthus, rest in the single genus Clypeaster. There 

 is thus no occasion to use Duncan's proposed genera Plesianthus and Diplothe- 

 canthus. Possibly a careful revision of the fossil forms may show some good 

 generic groups among them but for the recent species, a single genus will answer. 



Key to the Genera of Clypeastridae.^ 



Poriferous areas of petals divergent, not incurved dist ally; anus marginal Anomolanthus. 



Poriferous areas of petals more or less incurved distally; anus inframarginal .... Clypeaster. 



'In this and all subsequent keys only recent forms are considered. 



