38 ROBERT TRACY JACKSON ON ECHINI. 



has been seen, but in two cases oculars are separated more or less widely from the apical disc 

 (text^fig. 1, and Plate 7, fig. 2). In a Strongylocentrotus drobachiensis (text-fig. 1) a curiously 

 abnormal and instructive condition occurs. The test is pentamerous ventrally, but dorsally 

 the terminations of two ambulacra and four half-interambulacra are disjunct from the apical 

 disc. There are five genitals and three oculars in the apical disc, but two oculars, I and IV, 

 have become separated from the oculo-genital ring. The system is the same as that described 

 in Toxopneustes (p. 42; Plate 7, fig. 2), only, as the separation occurred later in life, the 

 distance of the oculars from the apical disc is not so great as in the Toxopneustes. Ocular IV 

 (text-fig. 1), is furthest removed. Ambulacrum IV is twisted dorsally, but extends ventrally 

 as usual. The right half of interambulacrum 3 abuts against ocular IV, and the left half 

 of interambulacrum 4 curls around dorsally so as to come in contact with the opposite side 

 of ocular IV. The left half of interambulacrum 3 and right half of 4 extend to oculars III 

 and V respectively as usual. A similar condition exists in the relation of the ambulacrum 

 and half interambulacra to the misplaced ocular I. If the abnormality had occurred earlier 

 in growth, this would have made a very striking specimen, but yet it elucidates the principles 

 involved quite clearly. In this specimen it is clear that ambulacra I and IV, with the associated 

 half interambulacra follow the oculars I, IV in their displaced position. 



Tetramerous. 



4. Four ambulacra, interambulacra, oculars, genitals, and teeth. More or less completely 

 four-rayed specimens are the commonest departure from the pentamerous system, yet com- 

 pletely four-rayed specimens were seen in only nine cases, all in three species. In a specimen of 

 Strongylocentrotus drobachiensis (Plate 6, fig. 9) there are four ambulacra, interambulacra, 

 oculars, genitals, and teeth, and eight primordial ambulacral plates in the peristome. It is 

 oriented by the madreporite and character of the bivium reaching the periproct, or insert, as 

 it is here called, which is a strongly dominant feature of the species. Accepting this orienta- 

 tion, the absent parts are ocular IV and its associated parts, which are ambulacrum IV, the 

 right half of interambulacrum 3 and the left half of interambulacrum 4. The missing genital 

 may be either 3 or 4, which, it is perhaps impossible to say. Four other specimens of Strongylo- 

 centrotus drobachiensis show the same structure. In all, the bivium is insert and the missing 

 parts are the same as in the specimen just described. One of these specimens is from Dumpling 

 Islands, Maine (R. T. J. Coll., 816), and measures 18 mm. in diameter. Two are from French- 

 man's Bay, Maine (R. T. J. Coll., 908, 909), and measure 42 and 52 mm. in diameter 

 respectively. The fourth specimen is from Truro, Massachusetts (R. T. J. Coll., 817), and 

 measures 26 mm. in diameter. In this specimen the lantern is wanting, but there are eight 

 auricles which indicate a four-parted lantern. Four specimens of Microcyphus maculatus, 



