232 R. T. JACKSON STUDIES OF PAL^ECHINOIDEA. 



is merely carrying this principle further, and I think is straining no argu- 

 ment, to say that the introduction of columns of 2)lates,both interambu- 

 lacral and ambulacral, in Melonites (plate 2, figures 2 and 4), Oligoporus 

 (plate 6, figure 25) and other types, indicate stages in growth through 

 which the individual has passed in its development. 



Accepting this principle, that stages in growth may be ascertained by 

 a study of the adult, it is necessary to see what are its limitations. It is 

 the plates themselves, and not their form, which are claimed as marking 

 stages. The outlines of plates during increase in size may suffer changes, 

 as discussed, but in Palseechini it does not seem that the}^ could have 

 altered much, for the form, esj^ecially of interambulacral plates, is very 

 definite in almost all cases (see Melonites, ])late 2, figure 2), and is just 

 that form which is required by the mechanical conditions of impact, 

 which conditions would have been nearly or quite the same in the young 

 as in the adult. The surface ornamentation of plates cannot probably 

 be used in the study of stages of an adult, as it is subject to much altera- 

 tion, as pointed out by Professor Loven (27). From a stud}" of the adult 

 we can ascertain the anatomy of the initial beginning of the corona, at 

 least in general terms, when no resorption has taken place, as in Echi- 

 narachnius and Lepidechiniis (plate 7, figure 42), or if the peristome has 

 by its advance resorbed the ventral border, we then have the ventral 

 border wanting to a greater or less extent, as in Melonites (plate 2, figure 3), 

 Aixhxocidaris, and Cidaris (plate S, figures 44, 45, 47, 48, and plate 9, 

 figure 55). 



Professor Loven in his ** Echinologica," page 18, describes the ambu- 

 lacral i)lates of echinoids as forming rapidly and from the pressure caused 

 by their own increase and that of the interaml)ulacra, the plates come 

 gradually to change their place relatively to the interradials and glide 

 downwards toward the stomal region, where by resorption they are dis- 

 charged, as it were, through the outlet of a river into the buccal mem- 

 brane. This gliding downward I confess I cannot see in any of the 

 types studied. If it took place, how could the primitive first row of 

 ambulacrals be retained around the oral orifice of an adult, as in Bothri- 

 ocidaris (figure 4, page 234) and in Arachnoides f Also, if this gliding down 

 took place, how could we in an adult find stages through which ambu- 

 lacral areas had passed in their development, as shown in Melonites (plate 

 2, figures 2 and 4), Lepidesthes (plate 9, figure 53) and Oligoporus (plate 

 6, figure 25) ? Or how could fan-shaped ambulacral plates exist at defi- 

 nite positions, as in Oligoporus missouriensis (plate 9, figures 50, 51) ? 



Ambulacral plates in echinoids, on account of their relations to the am- 

 bulacral tube-feet, seem to be more important structurally than interam- 

 bulacral plates, which latter, so to speak, are developed to fill interme- 



