986 MR. r. A. BATiiBB ON riNTACEHs^CTS. [Dec. 17, 



so low as IlBrj. The succeeding interdistichal usually stretches 

 across the interdistichal area, but it may be spHfc longitudiually iu 

 two. Also it may be succeeded by more interdistichals, from 1 

 to 5. The number of interdistichals is not necessarily constant in 

 the interdistichal areas of one individual. Considering this exten- 

 sive variation, it is a little odd that it should not have been noticed 

 by any of the American writers. Similar variation is not known in 

 the unique specimen of U. luestfalicus, which in this portion of its 

 anatomy agrees with tlie normal U. sociaUs. 



The interpinnulars are small plates that are often intercalated 

 in the spaces between the proximal portions of the fixed pinnules 

 and the intervening llEr. There is usually only one interpinuular 

 between two adjacent pinnules, but sometimes there are two. 

 An interpinnular on one side of an area has usually a corre- 

 sponding interpinuular on the other side. The interpinnulars are 

 quite common : I have seen them in about half of the specimens 

 under examination, e. g. y, e, d,f, g, Ic, I (figs. 4, 5, 8, 10, 11), and 

 it is a little curious that they are neither figured nor mentioned by 

 any previous writers. 



We have seen that the fixed pinnules probably arose from free 

 pinnules becoming involved in the construction of the cup. We 

 may inquire now as to the origin of the interbrachials and inter- 

 distichals. Are they derived in whole or part from pinnules, or 

 from the tegmen ; or are they mere supplementary plates ? The 

 variation in number might show that they are degenerate pinnular 

 elements; but it is equally consonant with the theory of their 

 supplemental origin. There is the very slightest trace of any 

 linear arrangement, and in such rare cases as might plausibly be 

 imagined to represent a degenerate pinnule, such an assumption 

 would upset the regular sequence of pinnulation ; so that one 

 cannot adopt this view without further proof. I can distinguish 

 no difference in this respect between young and old individuals. 

 The constancy of the proximal interbrachial might lead one to 

 regard it as a primitive tegminal element, since an iuterradially 

 situate plate of the tegmen is often conspicuous in just such a 

 position. But the proximal interdistichal, which can scarcely be a 

 primitive tegminal element, is nearly as constant. The presence of 

 interpinnulars, which are undoubtedly intercalated, supplementary 

 plates, show how all these plates may have arisen. Considered as 

 supplementary plates, the interbrachials may primitively belong to 

 either the cup or the tegmen ; one can regard all interbrachial 

 plates as portions of the tegmen that have become included in the 

 dorsal cup. 



The genex'al relations of all the component elements and areas 

 of the dorsal cup to one another are as follows : — The fixed primi- 

 brachs and secundibrachs form slightly raised ridges, more pro- 

 minent distalwards. The interbrachial areas, including inter- 

 brachials, fixed pinnules, and interpinnulars, are slightly depressed 

 at the sides, but form a slightly elevated shield-like area. The 

 interdistichal areas are depressed. These points are more obvious 



