38 BULLETIN OF THE 



appearance of homologous calcareous plates in genera closely allied to 

 each other, which certainly looks as if the difference in the time of 

 appearance of plates is no valid objection to a strict homology of those 

 plates. How much a recognition of this principle may change or modify 

 existing ideas of Echinoderm morphology, as far as the plates are con- 

 cerned, remains to be seen. It is possible that some of the differences 

 in the sequence of the plates of Asterias as compai-ed with other genera 

 of Asteroids may be explained in this way. If we recognize so much 

 difference in the time of the appearance of homologous plates in genera 

 closely related, does it not call for great caution in this particular in the 

 comparison of genera of different groups ] It does not seem too much 

 to say, that an acceleration or retardation in the time of appearance 

 of primary plates may have led to essential differences in the exter- 

 nal forms of Echinoderms. On the other hand, it is very strange if 

 geographical distribution has brought about sucli a great difference in 

 the sequence of plates as that which Carpenter finds between the Ameri- 

 can and European forms of A. squamata. It seems as if there must be 

 some mistake in the identification either of the European or American 

 specimens. As far as external form goes, my Newport specimens closely 

 resemble A. squamata, and specialists in the study of Ophiuraus have 

 so identified them for me. Shall we call the American and European 

 representatives different species or different genera, or does A. squamata 

 in Narragansett Bay depart so widely from the same in European waters 

 as far as development goes ] 



Dorsocentral. — There seems to be a uniformity of belief that the mid- 

 dle plate of the abactinal region of the body is homologous in Asterias 

 and Amphiura. The only essential point of difference is the presence of 

 a large spine in the young Asterias and its absence in Amphiura. This, 

 however, is not thouglit to be of importance enough to have any mor- 

 phological meaning. The author has no doubt that the dorsocentral 

 forms in the same relative position in both genera, as shown by the ob- 

 servations already recorded. Whatever objections, therefore, might be 

 urged on the ground that the sequence * is different, are not regarded 

 as fatal. The name dorsocenti-al is well chosen, but should not be 

 confounded with the centrodorsal of Crinoids. 



Genitals. — The author homologizes the first ring of five plates, which 

 form in the interradii of both Asterias and Amphiura, with each other. 



* The only plates of Asterias which can be homologized with the radialia of 

 Amphiura develop in Asterias ajler the dorsocentral. This fact probably has no 

 morphological meaning. 



