MUSEUM OF COMPARATIVE ZOOLOGY. 281 



nnd give us hereafter a much more accurate hasis in our limitation of 

 genera, species, and the higher subdivisions. But it would be out of 

 place here to do more than hint at this reform, especially as I trust soon 

 to publish, in our Illustrated Catalogue, a Revision of the Echini, 

 which has been undertaken, with the collections in the Museum and of 

 the Smithsonian as a basis. I shall always consider myself fortunate to 

 have had the opportunity — thanks to the liberality of the Superinten- 

 dent of the Coast Survey — of examining this collection, forming the 

 most valuable addition to our knowledge of recent Echinoids since the 

 collections of the same order made by Stimpson in tiie Pacific. 



In Toxopneustes drobachiensis Ac soon after resorption of* the Plu- 

 teus the young Sea-urchin has few large tubercles with mamelon, limited 

 to the ambitus (Podocidaris and Podophora-like). The next Mage has 

 two principal rows »f large tubercles occupying tl»d whole test (Cidaris- 

 like, no miliaries), increasing in number as they grow older, the spines 

 gradually passing from a condition similar to those of Rhabdocidaris, 

 Cidaris, Echinocidaris, and finally to Toxopneustes-like spine?, as fast 

 as the primary tubercles are formed, retaining their embryonic features 

 most strongly while the spines are directly connected to the test, as in 

 Podocidaris. In the earlier stages the actinal opening is large (Echino- 

 cidaris-like), without indentations (Cidaris-like), occupying nearly the 

 whole of the actinal surface. As the test increases this opening becomes 

 proportionally smaller, and slight cuts are formed (Psammechinus-like). 

 The anal system is at first closed by a single subanal plate, appearing 

 before the formation of the genital and ocular plates ; it remains for a con- 

 siderable period more prominent than the other plates, which are added 

 to cover the enlarged anal system. The symmetrical axis of the sub- 

 anal plate does not hold a fixed relation to the madreporic body, being 

 opposite different genital plates in various stages of growth. This cor- 

 responds to the oblique position of the subanal plate in Salenida\ 

 when we take as starting-point the madreporic body. The abactinal 

 system subsequently passes through a stage reminding us of Echinoci- 

 daris and Trigonocidaris, only there are five instead of four anal plates. 

 The poriferous zone is at first narrow, the pores arranged in vertical 

 rows ; subsequently they are slightly arched vertically ; they next sep- 

 arate into horizontal arcs of a smaller number of pores, increasing 

 rapidly in number with age, and in small specimens we can trace their 

 mode of formation, as the arcs near the ambitus are similar to those of 

 36 



