ECHINUS GRACILIS. 293 



ECHINUS. 



Echinus Rond. 1554. De Piscib. (Linn.), (emend.) 



This genus, as limited by Desor, contained only species more or less 

 globular, having comparatively small tubercles, smooth, imperforate, of nearly 

 equal size on the two areas, forming but two principal vertical rows upon the 

 coronal plates upon both areas, the other tubercles being smaller and irreo-u- 

 larly arranged. Actinostome small, with but slight cuts. Buccal membrane 

 bare, with only the ten buccal shields. Spines comparatively stout, fre- 

 quently attaining a considerable size, in some of the species equalling the 

 test in diameter. Pores arranged in arcs of three. Jaws comparatively 

 weak, auricles slender. 



Lutken has first thrown doubt upon the validity of this genus, and is in- 

 clined to consider the species of Psammechinus also as true Echini. If we 

 were to found this genus simply upon the presence or absence of the plates 

 upon the buccal membrane, his objections would be well founded. It may 

 be objected that we must take into account the facies of the tubercles, the 

 close arrangement of the miliaries and secondaries contrasted with the sparse 

 and regularly placed primary rows of the true Echini, and the totally dif- 

 ferent structure of the abactinal system, which would seem to justify us 

 in considering Psammechinus as a subdivision, certainly ranking as a sub- 

 genus of Echinus proper; yet in Echinus sphaera we find the young so 

 closely resembling the typical Psammechinus that we plainly see that the 

 objections to Lutken's proposition are not based upon solid ground. 



Echinus gracilis 



! Echinus gracilis A. Agass. 1869. Bull. M. C. Z. I., No. 9. 



PI. VI". f. 6; PI. VII. f. 1-6. 



This species holds an intermediate position between E. Flemingii Ball 

 and E. melo Lajl, to both of which it is allied. Like the former, it is sub- 

 ject to great Variations in the ratio of the longitudinal and vertical diameter 

 of the test. The primary tubercles are larger than those of E. melo, but 

 smaller than those of E. Flemingii. The spines in the proportion they bear 

 to test are similar to those of E. melo. The general pattern of coloration is 

 the same, consisting of bands of green made up of irregularly shaped lozenges 

 running in vertical rows, diminishing in intensity towards actinostome, the 



