384 Prof. S. Lovdn on the Structure of the Echinoidea. 



of the interradia from plate 1, which takes part in the forma- 

 tion of the peristome, which here, as in all these genera, is 

 complete, i. e. composed of ten ambulacral plates and five in- 

 terradial plates. 



Irregular forms, with a bivium which differs from the 

 trivium, are Mellita (Jiexapord) and Rotula {Rumphn), in 

 which plates 2 in I. a and V. b become enlarged inwards, and 

 therefore do not interrupt the unpaired interradium, of which 

 the sequence of plates is continuous ; whilst the former in the 

 trivium and in I. b and V. a has plates 2 and 3, and the latter 

 plate 2 in the trivium and plates 2 and 3 in I. b. and V. a, so 

 dilated that they form a ring which is open only at the un- 

 paired interradium, and by which the sequence of plates in the 

 four paired interradia is interrupted and in each of them peristo- 

 mial plate 1 separated from the following ones. EcMnarach- 

 nius j)arma and Lobopliora are irregular in a contrary way ; in 

 them plates 2 in I. a and V. b are more dilated than those in 

 the trivium and in I. b and V. a, so that the unpaired inter- 

 radium is interrupted in a much greater degree than the paired 

 ones. All these also have the peristome complete, composed 

 of ten ambulacral and five interradial plates. Arachnoid es is 

 singular in having the oldest ambulacral plates so strongly 

 dilated in breadth that in all five interradia plate 1 has dis- 

 appeared from the peristome, which consists only of the ten 

 first ambulacral plates, forming with the two or three following- 

 ones a broad connected ring, which throws the interradia far 

 away. But even here the bivium makes itself felt, although 

 in a small degree. Between plates 2 in I. a and V. b there is 

 a little space left open for two very small and compressed 

 plates of the unpaired interradium. 



Thus, whilst in the Echinidce, Echinoneus^ and the Cassi- 

 dulidse the test is still nearly regular and regularly divided 

 into ambulacra and interradia, and in the tSpatangidaj the 

 former only rarely predominate so as to displace the latter from 

 the peristome, in the ClypeastridiB the preponderance of the 

 ambulacra is the rule, and in most of them the interradia are 

 therefore interrupted. It is of importance to ascertain whether 

 these peculiarities pertain to the earliest ages or make their 

 appearance during growth. 



A Mellita hexajwra 6*5 millims. long has great distance 

 between the rows 1. a and V. J, so that the interradium between 

 them lies free and broader than in the others ; but plates 2 in \.b 

 and V. a of the bivium and in the whole of the trivium have 

 already become so widened tliat they form a connected circle 

 which excludes the following plates from the peristomial in- 

 terradial plate 1. It is, however, only at a still larger size 



