Terms in Cn'noid Morphology/. 5§ 



tology of New York,' vol. i. (1818). Tt seems to me there- 

 fore that the use of the term '' costals " in the sense now pro- 

 posed cannot be justified, and I regret that I ever agreed to 

 use it. 



The objective difficulties in the way of the proposed termi- 

 nology are due to the more correct views that arc now held 

 with regard to the homologies of pinnules. As was fully 

 explained in the section on the Arms in " British Fossil 

 Crinoids," Part II. (p. 371), pinnules are nothing more than 

 armlets that have become small, ceased to branch, and are 

 regularly placed on alternate sides of successive ossicles. An 

 armlet itself is merely one branch of a dichotomous arm 

 reduced in size. Consequently, from a morphological stand- 

 point, a pinnule, however small, is the homologue of a whole 

 diehotom (as we may conveniently call such a branch), while 

 •the ossicle that supports a pinnule is simply an axillary, and 

 this without going beyond the strict concei)tion of that term 

 as recently laid down by Carpenter {op. cit. p. 19). 



If now we turn to such a genus as Botri/ocrinus^ and com- 

 pare two of its species, such as JJ. ramosus and B. decadac- 

 ti/lus *, and if we name the successive orders of brachials after 

 Ihe methods hitherto followed, we shall come to these con- 

 clusions — that 



in B. ramosus in B. decadactylus 



the costals are homologous with the costals ; 



the distichals „ with the first two distichals ; 



the distichal axillary „ with the second distichal ; 



the palraars „ with the third disticlial and the first pinnule, 



or, if this pinnule is branched, with the 



proximal portion thereof ; 

 the fu-st postpalmars „ with thi^ fourth distichal, the second pinnule, 



and the branches of the first pinnule if it 



be branched ; 

 the second postpalmars „ with the fifth distichal and thb'd pinnule ; 



and so on. Which conclusions appear a sufficient redactio ad 

 ahsurdum of our present methods. Those methods were only 

 iigitimate so long as pinnules were considered to be struc- 

 tures distinct from arm-branches and present or not according 

 to some unrecognized or, at the best, empirical system. 



From the foregoing review of the circumstances it appears 

 that a terminology is required that shall fulfil the following 

 conditions. Homologous parts must receive the same name. 

 Parts serially homologous must receive names of a similar 

 liature. When specialization and difiPerentiation have taken 



. * Brit. Foss. Cria., V., Ann. & Mag. Nat. Hist. ser. 6. vol. vii. pp. 394 



and 398. ' " ' 



