ECHINOIDEA. II. 



specimens of any Echinothurid hitherto examined (leaving aside the very doubtful .Asthenosoma hystrix 

 of 3-i mm figured in Rev. of Ech., PI. II c.) are those of 3""" in diameter described by me (Parti, p. 174). I 

 have stated there that the periproct is, even in the smallest specimens, covered by a number of small 

 irregular plates, with no larger between. So a central plate seems never to be found here. Since in 

 this very young stage the anal plates are thus already present in considerable number and do not show 

 any trace of five original larger plates covering the whole anal area, I do not think we are justified 

 in assuming that these 5 large plates are found in a yet earlier stage. I give here a figure of the anal 

 area of the youngest specimen of Phormosoma (scarcely 3 mm ) seen by me. (Fig. i.) 



A matter of much more importance, however, is the statement (p. 91) that in Phormosoma 

 hispidum the bare interambulacral area adjoining the primordial plate is covered with a few minute, 

 elongate, irregularly arranged plates, which correspond to the interradial buccal plates of Cidaris . 



The same thing is stated for Kamptosoma indistinctum (p. 112): Iu this species we find a few of 



the same irregular elongate interambulacral plates which in the 

 Cidaridse are as well and as regularly developed as the ambu- 

 lacral buccal plates . It was hitherto assumed to be one of the 

 most important features distinguishing the Cidaridcr from all 

 the other regular Echinoids that both the ambulacral and inter- 

 ambulacral plates con tiiuie over the peristome; the Echmotlinridcr 

 were distinguished by the ambulacral plates alone continuing 

 over the peristome. If these small plates of the peristome found 

 in the two Echinothurids by Professor Agassiz were really 

 homologous to the interradial buccal plates of the Cidarida- this 

 fundamental character would have to be given up. Fortunately, 

 the figures given by Agassiz himself afford the proof that 

 these plates are not homologous with the interradial buccal 

 plates of the Cidarids; since the primordial interambulacral 



plate is persistent, these small plates lying in the buccal membrane inside (adorally) the primordial 

 plate cannot possibly have any relation whatever to the interambulacral plates and cannot be said to 

 ^correspond to the interradial buccal plates of Cidaris. They correspond to those small, irregular 

 plates found in the peristome of the other regular Echini. 



In treating the Echinothurids in Part I, I had to leave incertce sedis the species <s.Phormosoia 

 panamense and hispidum, and Professor Agassiz, not recognizing my limitation of the genus Phor- 

 mosoma, does not take the trouble to state to which group these species belong. But from the very 

 careful description and figures of the test combined with my examination of the pedicellarice of the 

 type specimens in the U. S. National Museum, it can be said with certainty that they belong to the 

 genus Echinosoma. It is true that the character of the primary actinal spines of panamcuse is unknown, 

 but all the other characters are decidedly those of Echinosoma, so that I think we may safely conclude 

 that the spines also are tipped with a hoof and not provided with a fleshy sack. A more detailed de- 

 scription of the pedicellariae I cannot give on this occasion; it will suffice to say that they agree rather 

 closely with those of Echinosoma uranus and tenue; in panamense I have not, however, found the 



Fig. i. Apical system of a young Pkormo 

 soma placenta, ^mm in diameter. 28 /i. 



