imbrication: genital and ocular plates. 155 



ambulacrum can not be said to show imbrication. They are very thick 

 in the species Melonites midtiporus (plate 2, figure 5), and thicker still in 

 Melonites giganteus, Jackson, as shown in the broken portion of area I 

 (plate 4, figure 19). The several sides of the plates diverge upward and 

 outward as a mechanical necessity to fill space in a curved test. In 

 species where the test is thin, as in Oligoporus coreyi, M. and W., the 

 wedge-like form of the plates is less apparent than in species where the 

 plates are thicker as described. 



The details of imbrication or want of it as described in Melonites multi- 

 porus have been also observed in Oligoporus danse and Rhoechinus elegans. 

 Therefore this type of mutual plate contact may be considered as char- 

 acteristic of the family. 



Genital and ocular Plates and Discussion of Orientation. 



In the Johns Hopkins specimen of Melonites multiporus (plate 3, figure 

 13) the genital plates are seen in all five areas. In these plates there are 

 three, or four pores in each plate. The same condition occurs in a speci- 

 men in the Museum of Comparative Zoology (catalogue number 3077), and 

 in a specimen in the Boston Society of Natural History (catalogue num- 

 ber 11569), in both of which five genital plates are preserved intact. In 

 no case have five pores or less than three pores been observed in a genital 

 plate, though altogether quite a large number of plates have been exam- 

 ined. The same condition of pores has been observed in Oligoporus 

 missouriensis, Jackson, as described in the succeeding paper. 



Messrs Meek and Worthen (30) have figured the genital and ocular 

 plates of Melonites midtiporus, and in their figure each of the genital plates 

 has four or five pores. We would not question the accuracy of their 

 figure, for all the work of these authors on Paleozoic echini is of the most 

 painstaking and accurate kind ; but it is felt that this large number of 

 pores is not to be considered the normal or average condition, being 

 rather an unusual increase, Mr C. R. Keyes (23) has recently published 

 a figure of Melonites inultiporus, in which the genital plates have three 

 pores in two cases and four in two others. The fifth plate has a single 

 large pore, but has a number of minute ones, and is described as a madre- 

 poric plate. We have not seen any genital plate with less than three 

 pores and have never seen evidence of a madreporic character in any of 

 the plates. Some one of the plates, from analogy, one would suppose 

 must be madreporic in nature, and it is important that Keyes has shown 

 it may be found, although specimens showing it must be very rare. It 

 seems possible that there is in his figure some error about the single large 

 pore ; but if it is correct, then a reduction as well as an increase from the 

 normal number must be allowed for. 



