THE PENTAMKROl'S SVSTKM AND VARIATION 



genitals 1 and 2, has in its abnormal position grown to :i large ami >ome\vliat irregularly r-hapod 

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

 which finds its dorsal limit in this plate. Also extending vent rally from ocular II then- arc 

 two half-interambulacra, the right half of interambulacrum 1 ami tin- l.-ft half of interaml.ula- 

 crum 2. Proceeding dorsally from ocular II, the coronal plat.- that an- typically awoeiated 

 with this plate cease, but the left half of interambulacrum 1 ami the right half of interaml.ula- 

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

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

 not congenital. It is similar in principle to text-fig. 1, p. 37, but is more striking :is 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-intcrambulacra (compare text- 



fig. 218, p. 191). 



13. Pentamerous ventrally, four ambulacra and inlerambulacra dorsally, J 



genitals. A Strongylocentrotus drobachiensis from York, Maine (R. T. J. Coll., 823), 40 mm. in 

 diameter, is pentamerous ventrally, but ambulacrum II consists of only a pair of |.l 

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

 highly reduced. Above this extreme ventral area there are only four ambulacra an. 

 ambulacra. The ocular associated with ambulacrum II has quite disa PI >earel. pmbabl 

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

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

 structure is shown in an Echinus mageUanicus from the Straits of Magellan in the . 

 of the United States National Museum. There are five teeth and ten prunor 

 lacral plates. In the corona there are five ambulacra and interambulacr 

 ambulacrum I and the associated interambulacral plates exist for only a short du 

 which there are only four areas. There are four oculars and five gcnit; 

 1 and 5 are depauperate and imperforate. 



14 Pentamerous ventrally, four ambulacra and inlerambulacra dorsntl,, 

 genitals.- A Strongyhcentrotus drobachitnsis from Dumpling Islands. M, 

 824) 18 mm. in diameter, is completely pentamerous ventrally. 



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

 Ocular V has quite disappeared. There are four oculars and genitals in p h 

 differs from the last considered only in that there are four instead of 



15 Pentamerous ventrally, four ambulacra and five interambulacra dorsal!,. J, 

 s-TKs is the commonest type of departure from the complete ,x-nta 



aena- 



Id, 17 cases having been seen. A small specimen of Stron^loc.^,, 

 (Plate 7 fig 4) shows interesting features. The specimen is 17 mm. m dmmeU 



