THE PExNTAMEROUS SYSTEM AND VARIATION. 43 



genitals 1 and 2, has in its abnormal position grown to a large and somewhat irregularly shaped 

 plate (compare oculars in text-fig. 1, p. 37). Below this ocular extends ambulacrum II, 

 which finds its dorsal limit in this plate. Also extending ventrally from ocular II there are 

 two half-interambulacra, the right half of interambulacrum 1 and the left half of interambula- 

 crum 2. Proceeding dorsally from ocular II, the coronal plates that are typically associated 

 with this plate cease, but the left half of interambulacrum 1 and the right half of interambula- 

 crum 2 fill the space and extend respectively to ocular I and ocular III, their normal point of 

 origin. There are five teeth. This specimen owes its peculiarities to relatively late influences, 

 not congenital. It is similar in principle to text-fig. 1, p. 37, but is more striking as the ocular 

 is farther removed from its normal position in the apical disc. It lends weight to the view 

 held that ocular plates are a controlling influence in the development of the corona, and that 

 from each ocular originates an ambulacrum and two half-interambulacra (compare text- 

 fig. 218, p. 191). 



13. Pentamerous ventrally , four ambulacra and interambulacra dorsally, four oculars, and five 

 genitals. — kStrongylocentrotus drdbachiensis from York, Maine (R. T. J. Coll., 823), 40 mm. in 

 diameter, is pentamerous ventrally, but ambulacrum II consists of only a pair of plates in the 

 basicoronal row. The associated two half-interambulacra of areas 1 and 2 are similarly 

 highly reduced. Above this extreme ventral area there are only four ambulacra and inter- 

 ambulacra. The ocular associated with ambulacrum II has quite disappeared, probably by 

 resorption. Four oculars and five genitals are in place in the apical disc. Genital 1 is 

 depauperate and lies against the madreporite. There are five pairs of auricles. A similar 

 structure is shown in an Echinus magellanicus from the Straits of Magellan in the collections 

 of the United States National Museum. There are five teeth and ten primordial ambu- 

 lacral plates. In the corona there are five ambulacra and interambulacra ventrally, but 

 ambulacrum I and the associated interambulacral plates exist for only a short distance, above 

 which there are only four areas. There are four oculars and five genitals of which genitals 

 1 and 5 are depauperate and imperforate. 



14. Pentamerous ventrally, four ambulacra and interambulacra dorsally, four oculars and 

 genitals. — A Strongylocentrotus drobachiensis from Dumpling Islands, Maine, (R. T. J. C-oll., 

 824), 18 mm. in diameter, is completely pentamerous ventrally. A slight distance above the 

 ventral border ambulacrum V drops out and also the two associated half-interambulacra. 

 Ocular V has quite disappeared. There are four oculars and genitals in place. This type 

 differs from the last considered only in that there are four instead of five genital plates. 



15. Pentamerous ventrally , four ambulacra and jive interambulacra dorsally. Jive oculars and 

 genitals. — This is the commonest type of departure from the complete pentamerous system 

 found, 17 cases having been seen. A small specimen of Strongylocentrotus drobachiensis 

 (Plate 7, fig. 4) shows interesting features. The specimen is 17 mm. in diameter, 9 mm. high, 



