ROBERT TRACY JACKSON ON ECHINI. 



described were evidently cases of congenital variation, whereas the next type considered is a 

 variation to six parts taken on at a comparatively late stage in development. 



The specimen of Strongylocentrotus lividus described by Ribaucourt (1908) would belong 

 in this series, as he says that it is completely hexamerous, having six teeth, six ambulacral and 

 interambulacral areas, six oculars and genitals. It would be interesting to know in which 

 area the modification takes place and if there is a fusion of genitals as in the specimens above 

 described. 



19. Pentamerous ventrally, six ambulacra and interambulacra dorsally, six oculars, five 

 genitals.- A most interesting condition occurs in a specimen of Tripneustes esculentus found 

 in material kindly collected for me in Jamaica by Dr. Thomas Barbour. This choice speci- 

 men (Plate 6, fig. 4) measures 76 mm. through III, 5 and is 45 mm. in height; it presents a 

 slight hump in the aberrant area. There are six oculars, but only five genitals. The addi- 

 tional sixth ocular lies between ocular II and the madreporite. Ventrally the specimen is 

 pentamerous and quite normal, but above the mid-zone a sixth ambulacrum appears interca- 

 lated between ambulacrum II and interambulacrum 2. It extends dorsally, the two columns 

 of plates reaching ocular VI in the usual fashion. About halfway, up the sixth ambulacrum a 

 sixth interambulacrum appears. It consists of a single column of narrow plates which extend 

 to oculars II and VI. The single column and ocular contact are comparable to the character 

 seen in Bothriocidaris archaica (Plate 1, fig. 2). I believe it and the next described case are 

 the only known instances since Ordovician times of an interambulacrum consisting of a single 

 column of plates throughout its length. At two points the single column of interambulacral 

 plates is separated dorso-ventrally, and at these breaks ambulacra II and VI are in contact 

 for a brief space. Genital 3 is split horizontally, as in text-fig. 190. 



A like partially hexamerous condition exists in a Strongylocentrotus drobachwnsis from 

 Truro, Massachusetts (R. T. J. Coll, 836), 35 mm. in diameter. It is quite normal ventrally, 

 but dorsally there are, as in the above case, five genitals and six oculars. Ocular VI lies between 

 ocular V and genital 5 and is fused with the latter. A sixth ambulacrum for a length of 10 

 mm. lies between ambulacrum V and interambulacrum 5, the condition being as in Plate 6, 

 fig. 4. A sixth interambulacrum, consisting of two plates in a linear series, lies between 

 ambulacra V and VI as in the same figure. 



A small and distorted specimen of Arbacia punctulata (R. T. J. Coll., 882) kindly sent me 

 from Johns Hopkins University through Dr. G. T. Hargitt, again shows a similar structure. 

 The specimen measures 36 mm. in diameter through 4, I and 19 mm. in height. Pentamerous 

 ventrally, dorsally a sixth ambulacrum exists for a distance of 6 mm. The additional area 

 lies between interambulacrum 1 and ambulacrum II (compare Plate 6, fig. 4). A sixth inter- 

 ambulacrum consisting of two plates in a vertical series lies between ambulacrum VI and 

 ambulacrum II, as in Plate 6, fig. 4. There are six oculars and five genitals, but the latter are 



