140 JACKaON AND JAGGAR — STUDIES OF MELONITES MULTIPORUS. 



will be readily followed. It is to be observed that successive columns, 

 passing dorsally, are typically alternated to the right and left of one 

 another (plate 3, figure 12 ; plate 4, figure 18). This rule has some ex- 

 ceptions, principally in even-numbered columns, but occasionally in odd 

 ones. It will be seen that the plates of the adambulacral columns are 

 pentagonal, while all the other plates of the interambulacral areas are 

 hexagonal, with the exception of the ventral terminal plates of columns 

 which are pentagonal* and adjacent plates which are in most cases 

 heptagonal ; also near the anal area the plates, instead of being hexag- 

 onal, are more or less rhombic in form Tplate 3, figure 13). 



The arrangement as sketched above has been traced in more than 100 

 specimens of Melonltes mtdtlporus, and in all the same method of intro- 

 duction and growth is maintained, with only such slight variations as 

 are discussed. A similar arrangement has also been traced to a greater 

 or less extent in Rhoechinus, Palfeec/ilnus, Ollgoporus, Lepidechimts, Lepi- 

 docentrus, Lepldocidarls and Arch<neocldaris, and in a published figure of 

 Lepidesthes, as described in tlie succeeding paper. It may be stated that 

 no conflicting evidence has been found in any Paleozoic or recent type. 



Structure and Development of the Ambulacrum. 



Turning to the detailed description of plates, the oral area will be con- 

 sidered first. Specimens showing perfectly the oral termination of the 

 corona seem to be rare, for that area has never been adequately de- 

 scribed, and most specimens which are at first sight complete are want- 

 ing in one or more rows of plates. A specimen showing this area satis- 

 factorily is a Melonites multlporus, Norwood and Owen, from the Saint 

 Louis group, Subcarboniferous of Saint Louis, Missouri.! The specimen 

 is in the collections of the Museum of Comparative Zoology (catalogue 

 number 3000), and is represented in plate 2 by figure 2. The ambulacra 

 ventrally in each area consist of a row of four plates which lead to per- 

 pendicular columns by the addition of plates dorsally. The plates at 

 the ventral termination as well as the later added ones have two pores. 

 In each area, as also observed on many other specimens, the two outer 

 initial plates, a b (plate 2, figure 4; plate 4, figure 18), become the bases 

 of the two lateral columns of the ambulacrum, and the two middle initial 

 plates, a' h\ become the bases of the two columns of large median ambu- 

 lacral plates. At the ventral area all the ambulacral plates are of about 

 the same size and shape, whereas further up the plates of the two central 

 columns, a! h' , are much larger than the plates of the lateral columns 



♦Excepting the initisvl plate of column 3. See piate 2, figure? 2 and 3. 



fAll the specimens of Melonites multiporus described in this paper are froin^ the same formation 

 and locality; therefore no further details on this point will be given. 



