STRUCTURE AND DEVELOPMENT OF LEPIDECHINUS. 227 



not so regularly pentagonal as in Melonites and similar forms. In the 

 third row of Lepidechinus the third column of plates originates in plate 

 3 and forms the first added median column of interambulacral plates. 

 In the next row the fourth column is introduced by plate 4, which has 

 2 columns of plates on the left and 1 on the right, as usual in Melonites. 

 In the fifth row the fifth column is introduced by plate 5, with 2 columns 

 on either side. Initial plate 5 impinges on the dorsal side of initial 

 plate 3. The sixth column is introduced so quickly that its initial 

 plate 6 extends down into and becomes part of the fifth row. The seventh 

 column is also introduced very quickly, its initial plate 7 truncating the 

 dorsal border of plate 5. The eighth and ninth columns originate almost 

 simultaneously in plates 8 and 9, which both impinge on the dorsal border 

 of initial plate 7. It is to be observed that in Lepidechinus the initial 

 plates of columns are not pentagonal with adjacent heptagonal plates, as 

 in Melonites and most other genera of Paleozoic Echini, the scale-like 

 imbricating character, together with the rapid introduction, seeming to 

 do away with the usual forms of the plates. From the initial plate 1 

 to the introduction of the fourth column in plate 4, the development of 

 Lepidechinus (plate 7, figure 42), excepting for the scale-like form of its 

 plates, is like that traced in Melonites and Oligoporus. The later added 

 columns of Lepidechinus, however, present a striking difi'erence in their 

 extremely rapid introduction, surpassing any other known form of Palae- 

 echini, except Lepidocentrus mulleri (figure 2, page 223). In this feature 

 it may properly be considered a highly accelerated type, very early acquir- 

 ing features which appear much later in the nearest allied forms. 



The ambulacral plates of the corona of Lepidechinus rarispinus (plate 7j 

 figure 42) are in 2 columns, each plate of which extends quite across its 

 owm half ambulacrum. The plates have 2 pores and a prominent pit, 

 representing a spine tubercle. Smaller tubercles may have existed, but 

 they were not made out. On the peristome faint traces of the ambulacral 

 plates can be made out, as shown in the figure ; they are principally indi- 

 cated by the pores and spine tubercles. 



Prominent buccal pyramids are present orally, lying opposite each in- 

 terambulacral area, but the actual teeth are not shown. 



There are 2 specimens of Lepidechinus rarispinus in the American Mu- 

 seum of Natural History, New York, and they are the original types from 

 which the species was described. These specimens are smaller and have 

 smaller plates than the specimen just described. They show 11 columns 

 of plates, as originally described, and the number of columns increases 

 rapidly in passing from the ventral border dorsally, as in Professor 

 Beech er's specimen. I cannot say, not having observed it at the time. 



