ECHINOTHURIDiE. 151 



the relationship between the Echinothuridae and Diadematidae is very close. 

 We cannot believe that the many points of resemblance are either coinci- 

 dences or examples of parallelism. On the other hand, it seems clear that 

 many of the differences, such as those in the abactinal system and test, are 

 due to the increased size of the Echinothurids accompanied by decrease in 

 calcification. The relationship between Astropyga and Micropyga on the 

 one hand and Araeosoma on the other is very close ; were it not for the 

 difference in the actinostome they would certainly be regarded as belonging 

 in a single family. The recognition of a separate suborder (Streptoso- 

 mata) for the Echinothuridae, based on the flexibility of the test, seems to 

 us quite unnecessary. The test of several Echinothurids is little or no 

 more flexible than that of Astropyga and Micropyga, and it is certainly an 

 exaggeration of the differences between these genera and the Echinothu- 

 ridae to place them in different suborders. The Aspidodiadematidae, Dia- 

 dematidae, and Echinothuridae form a natural group with some interesting 

 primitive characters, and if suborders of the Diadematoida are to be recog- 

 nized they should certainly form one together. 



The Genera and Species of Echinothurids. 



The Echinothuridae are a fairly homogeneous and well-defined group, the 

 limits of which are so clear that there has never been any question raised 

 as to whether a given recent species were an Echinothurid or not. For many 

 years, only two genera (Phormosoma, Asthenosoma) were recognized, but 

 in 1897 Koehler described a new form (Sperosoma), the actinal ambulacra 

 of which are quite unique. In 1903 Mortensen split the family into ten 

 genera, and suggested the possibility of two more ; while he gave attention 

 to the structure of the test, his classification was based chiefly on the 

 characters furnished by the spines and pedicellariae, especial emphasis 

 being placed on the latter. Three of his genera (Calveria, Hapalosoma, 

 Tromikosoma) are based almost exclusively on the pedicellariae, and he 

 has suggested in later writings that they might not be maintained, a sug- 

 gestion in which we fully concur. Mortensen says " it is the spines, the 

 pedicellariae, the tube-feet, and the spicules which bear the principal part 

 in the new classification of the Echinothurids." " Of course also the struc- 

 ture of the test is always of importance, but the all predominant importance 

 that has hitherto been attached to the form and mutual relation of the 

 plates will have to be very much reduced." — "The arrangement of the 



