PERISCHOECHINIDAE. 649 



In Palaechinua (Fig. ?) we find but two rows of ambulacral plates, with four 

 rows of interambulacral plates, — differences which do not remove so abso- 

 lutely the Palaechinidae from the Echinothnriae, and show a more intimate 

 relationship between the carboniferous and the cretaceous mailed Echini than 

 has been supposed to exist heretofore. 



In Lepidestb.es (Fig. 4) the ambulacral areas are wider than the interam- 

 bulacral, and composed of a greater number of rows of plates ; while in 

 Melonites (Fig. 2) the ambulacral and interambulacral fields, though of 

 different width, are composed of nearly the same number of vertical rows 

 of plates. 



These examples of the Perischoechinidae have been taken as the most 

 prominent, and most fitted to show the af- 

 finities of this remarkable group of Echini 

 to the regular Echini. 



It may perhaps not be superfluous to 

 recall that the plates of the poriferous zones 

 of regular Echini (Podophora, Arbacia, some 

 species of Strongylocentrotus, Stomopneus- 

 tes, etc.) are frequently, in the immediate vicinity of the aetinostome, so 

 closely packed together as to pass, from simple -zones arching round the tu- 

 bercles on the abactinal side of the test, to broad, flat surfaces, over which 

 the pores are scattered without any apparent order, forming many rows 

 of ambulacral plates, irregularly packed (PI* III d . f. 5 ; Pi. I s - f. 6 ; PL 

 IV b . f. 1 ; PL V\ f. 2.), — the flattened petaloid actinal ambulacra in the 

 regular Echini. 



In fact, when we come to analyze more fully the mode of formation of the 

 ambulacral plates we shall find in many genera, — Stomopneustes, Strongy- 

 locentrotus, Heterocentrotus, which will be found noticed in the chapter on 

 the water-system, — that in the ambulacral system of the Desinosticha it is 

 possible to have several rows of ambulacral plates on each side of the median 

 line, much as we find them normally developed in the Perischoechinidae ; 

 forming, however, in the latter, usually vertical rows of pores, while in the 



of the ambulacral pieces (a) near the middle of the body, with the two pores penetrating the middle of each 

 piece, and in a few of those above the granules covering the whole surface of all the pieces. On the left 

 the interambulacral pieces (i a) are seen, showing their arrangement, and the comparative narrow breadth 

 of the interambulacral areas. By the side of these plates b represents a section of them, illustrating their 

 imbricating arrangement. On the right side of the ambulacral series, only the marginal row of the inter- 

 ambulacral plates is represented. 



Fig. 3. Oligoporus Danae (two diameters). 



