22 On the Anatomical Nomenclature of EcMnoderms. 



by Fewkes ; but he would have been more in accordance with 

 the rational terminology now current if he had used inter- 

 mediary or interambulacral for interbrachial*, and had adopted 

 Miiller's use of adambulacral. 



On p. 105 of his last paper f we read, " The plates in the 

 Echinoids called adambulacrals which lie between the system 

 of plates generally known as ambulacral are regarded as the 

 same as the marginal plates of the starfish." This passage 

 can only refer to the plates which are always called inter- 

 ambulacrals in the test of an Urchin, and I have been itnable 

 to discover that any author, except Fewkes, has ever called 

 them adambulacral. Ludwig J, however, has pointed out 

 that the ambulacral plates of an Urchin are in all probability 

 homologous with the adambulacrals of a Starfish, and in his 

 diagram of the skeleton of an Echinoid he marks these plates 

 " Adambulacralia (sog. Ambulacralia)." Fewkes, on the other 

 hand, speaks of " the so-called adambulacrals of sea-urchins " 

 when he means the interambulacrals, auct., although no pre- 

 vious writer has employed the term in this sense, so that there 

 was no reason for Fewkes to have done so. He accepts 

 Ludwig's homology of tliese interambulacral plates (adambu- 

 lacrals, Fewkes) with the marginals of the Starfish, as shown 

 in the following table, copied from p. 106 of his memoir : — 



Starfish. Sea-Uechin. 



1. Ambulacral rafters. 1. Wanting. 



2. Peripheral ambulacrals §, gene- I ,^ Ambulacral* 



rally called adambulacrals. j *" 



3. Marginals. 3. Adambulacrals. 



But he also remarks in a footnote, " The homologies here 

 presented are essentially the same as those already published 

 by Ludwig as far as the relationship between the ambulacrals 

 of the starfish and the adambulacrals of the sea-urcliin is con- 

 cerned.^' There seems to be something wrong here, for it is 

 clear that the adambulacrals of an Urchin cannot be homolo- 

 gous both with the ambulacrals of a Starfish (footnote) and 

 also with its marginal plates (table). 



It may be that a clerical error has been committed, the 

 prefix ad being put in the wrong place in the footnote, and 



* Tbi.s term is not particularly applicable in the case of Goniaster and 

 similar forms. 



t Proc. Boston Sec. Nat. Hist. 1889, vol. xxiv. p. 105. 



X " Entwickluiigsgeschichte der ^5<«'i7;a ///W;o*Y?, Forbes/' Zeitschr. f. 

 wiss. Zool. ]882, Bd. xxxvii. p. 73. 



§ These "peri):heral ambulacrals'' are also called interambulacrals by 

 Fewkes, and in his figure on p. 99 they are lettered ad, Adambulacrals f ! 



