52 Proceedings of the Royal Physical Society. 



Pentaceros sp. 



In the collection there are three small specimens which are undoubtedly 

 immature, but which belong to the genus Pentaceros. After a careful 

 comparison of the type specimens of the various species of this genus 

 in the British Museum, we have come to the conclusion that they approach 

 nearest to P. nodulosus, Perrier, but in view of the fact that they are young 

 and immature we would refrain from definitely referring them to this 

 species. 



The following notes may, however, be useful in illustrating the chief 

 points of similarity and contrast. 



The present specimens differ from the British Museum specimens on 

 which Perrier founded the species P. nodulosus (Archiv. de Zool. Expir., v., 

 1876, p. 53) in the following respects : — 



P = 67 r=24 P = 2-8r 



A' = 59 r = 21 E = 2-8r 



A=56 r = 21 B = 2-3r 



Perrier gives R = 2\r. 



Marginal plates : — Type specimen, . 36 to 38. 

 Present specimens, . 15 to 17. 



Tubercles on the lophial line : — Type specimen, . Ellipsoidal. 



Present specimens, . ' Dome-like. 



In addition to this we may note that in our specimens there are tubercles, 

 conical or dome-like, on the lateral and supero-marginal plates. On the 

 lateral plates they become larger towards the disc, while, in contrast to 

 this, on the supero-marginal plates they are more prominent towards the 

 tip of the arm. 



In larger specimens of P. nodulosus these lateral lines are not to be 

 seen, and it is possible that those of the specimens before us would disappear 

 with the growth of the individual. In want of evidence regarding the 

 possibility of this, we think it more advisable not to identify our specimens 

 witli this species at present. In the living animal the tubercles are light 

 yellow, and the rest of the dorsal surface is dark brown. 



The position of the madreporite is the same as in the type specimen, but 

 whereas in P. nodulosus it is lozenge-shaped, in the present forms it is 

 triangular to heart-shaped. 



