36 ROBERT TRACY JACKSON ON ECHINI. 



an ambulacrum originates on the ventral border of an ocular, each interambulacrum may be 

 considered as composed of two halves, the plates of which originated on the left or right of the 

 area in contact with the adjacent oculars (text-figs. 217, 218, p. 191). If this is true, then the 

 loss of an ocular would cause a failure to develop of the plates that normalh' went with it, also 

 an abnormal position of an ocular (Plate 7, fig. 2) should cause an abnormal distribution of 

 the associated coronal plates. 



The cases of departure from the pentamerous system are taken up in the order of their 

 structural characters. They are described in at least one case of each class and other cases 

 are noted. The cases fall under twenty more or less distinct combinations of characters, and 

 to facilitate the distinctions, they are numbered under so many distinct heads. 



Trimerous. 



1. Three ambulacra, inter ambulacra, oculars, and genitals, four teeth. — A specimen of 

 Strongylocentrotus drobachiensis collected at Dumpling Islands, North Haven, Maine, measur- 

 ing 5.5 mm. in diameter (R. T. J. Coll., 812), is the nearest approach to a completely trimerous 

 sea-urchin known. There are three ambulacra and interamlaulacra throughout the corona. 

 There are also three oculars and genitals. The oculars all reach the periproct, an extraordinary 

 character in the species, but no rules can be expected to apply in such a freak. There are four 

 teeth and eight primordial ambulacral plates in the peristome. 



2. Pentamerous ventrally, three ambulacra and five interambulacra dorsaUy, five oculars and 

 genitals. — It is quite a frequent occurrence for one ambulacrum to fail to reach the apical disc, 

 but rarer for two ambulacral areas to drop out. In a specimen of Strongylocentrotus drobach- 

 iensis from Dumpling Islands, 43 mm. in diameter (R. T. J. Coll., 813), the test is quite normal 

 ventrally, but dorsally ambulacra III and IV drop out (similarly, but later than in Plate 7, 

 fig. 4), so that interambulacra 2, 3, 4 come in contact dorsally and make a continuous series 

 of plates. All five interambulacra continue to the apical disc. There are apparently five 

 oculars, but III and IV are imperforate and merged in a series of split genitals. There are 

 five genitals, but genital 3 is imperforate and much split up by secondary sutures. A Strongy- 

 locentrotus drobachiensis from Frenchman's Bay, Maine (R. T. J. Coll., 907), which measures 

 53 mm. in diameter, is pentamerous ventrally but ambulacra I and V drop out below the mid- 

 zone. All oculars are in place but I and V are imperforate. As a result of this structure 

 interambulacra 1, 5, 4 are confluent dorsally and the six columns of plates of these three 

 areas extend to oculars II, I, V, IV as usual. 



A specimen of Toxopneustes atlanticus from Bermuda (R. T. J. Coll., 814) is quite similar. 

 It is 52 mm. in diameter and ventrally normally pentamerous. Ambulacra III and IV fail to 

 reach the apical disc by a distance of 10 mm., so that interambulacra 2, 3, 4 are in contact 



