44 ROBERT TRACY JACKSON ON ECHINI. 



and misshapen on the abnormal side. \'entrally there are five ambulacra, interambulacra, 

 and teeth, but ambulacrum II is developed for a short distance only. Dorsally there are five 

 genitals and oculars, but ocular II is imperforate. The dorsal border of the ambulacrum is 

 widely separated from its ocular. Ocular II lies dorsally and produces its share of interam- 

 bulacral plates, although ambulacral plates ceased to be built. Columns 1 and 2 of interam- 

 bulacrum 1 extend dorsally and abut against oculars I and II as usual. In interambulacrum 

 2, columns 1 and 2 extend dorsally, but only 2 reaches the apical disc, column 1 dropping out, 

 so that in the last three rows or zones of growth there is only a single column of plates in this 

 area, a condition somewhat comparable to that seen in area 4 of Arbacia (Plate 4, fig. 11) 

 where also a single column of plates exists dorsally. Just above ambulacrum II in this speci- 

 men (Plate 7, fig. 4) there are extra interambulacral plates that cannot be definitely referred 

 to either areas 1 or 2. They are virtually space fillers, which, in my opinion, is what the 

 interambulacral plates are. This specimen is structurally almost identical with the interesting 

 Echinus esculentus described by Ritchie and Mcintosh (1908). In both, the ocular remained 

 in place dorsally but imperforate, the ambulacrum existed ventrally only, and the adjacent 

 interambulacra developed in a comparable manner, though with certain differences. In the 

 Echinus only the four columns of the two interambulacra exist above the imperfect ambulacrum, 

 and extend dorsally to their respective three ocular plates. 



A Strongylocentrotus drobachiensis (Plate 6, fig. 6) has five oculars and genitals as usual; 

 there are five interambulacra continuous to the apical disc, but one of the five ambulacra, 

 IV, dies out dorsally, so that interambulacra 3 and 4 come in contact and alone touch ocular 

 IV. The distance of separation of the ambulacrum from its ocular is only slight, but it repre- 

 sents a zone in which there are only four ambulacra. In this specimen ocular III is split, a 

 rare variation, and genital 5 is divided. A few pores in the dorsal part of interambulacnun 3 

 are apparently supernumerary genital pores that lie below genital 3. 



An Arbacia punclulata, from Woods Hole, Massachusetts, in the American Museum of 

 Natural History, is peculiar in that ambulacrum III fails to reach the apical disc so that there 

 are only four ambulacra in that region. The structure is almost the same as in Plate 6, fig. 6, 

 except that a different ambulacrum is involved. Six specimens of Strongylocentrotus col- 

 lected at Dumpling Islands, Maine, (R. T. J. Coll., 825-830), one from Calderwood Island, 

 Maine (R. T. J. Coll., 845), one from Frenchman's Bay, Maine (R. T. J. Coll., 846), and two 

 from Friday Harbor, Puget Sound (R. T. J. Coll., 903, 904), a Strongylocentrotus eunjthrogravimus 

 from New South Wales (R. T. J. Coll., 905), also a Tripneustes esculentus from Hayti (R. T. J. 

 Coll., 831), and two specimens of Toxopneustes atlanticus from Bermuda (R. T. J. Coll., 894, 

 897), all show a similar structure to Plate 7, fig. 4, with five genitals and oculars in place and 

 an ambulacrum dropping out at a greater or less distance from the apical disc. In all but 

 one of these cases the ocular lying above the imperfect ambulacrum is imperforate. In the 



