MUSEUM OF COMPARATIVE ZOOLOGY. 1>7 



minded of the general aspect of Pygorhynchus. There are but two 

 principal rows of large tubercles in each area, extending from apex to 

 mouth, so that, seen from above, the young Echinarachnius has much 

 the facies of an Echinometra. The mouth is large, pentagonal, its 

 radius being half the radius of the test. The ambulacral rosette is 

 reduced to two pairs of pores, — - simple perforations of the test, one in 

 each poriferous zone for each ambulacrum. This extraordinary shape 

 and structure the young do not retain long ; they soon become pyriform ; 

 the blunt extremity being the posterior, the test becomes greatly flattened 

 and the anus approaches the edge. The rosette is now composed of 

 three and two pairs of simple pores in each poriferous zone for each 

 ambulacrum, the anterior ambulacrum having only two pairs in each 

 zone. The tubercles are proportionally smaller, though there are still but 

 two rows in each area, but further apart. In the next stage we find the 

 rudimentary rosette composed of four and five pairs of pores close to- 

 gether and two or three distant pairs of pores, in the following ambula- 

 cral plates, one pair in each plate, which in subsequent stages increase 

 in number and extend almost to the edge of the test. The test has be- 

 come quite flattened, the lower side is concave, undulating, the ambu- 

 lacral zones are now much narrower than the interambulacral ones. 

 Each plate still has only one tubercle ; the lines of separation between 

 the two zones run straight from the edge of the test to the apex. It is 

 only in somewhat older stages, when the rosette loses its radiating outline, 

 and assumes a slightly petaloid shape, that we find the angle formed at 

 the base of the petal in the ambulacral zone, from which point the 

 ambulacral plates widen rapidly ; each plate now carries from two to 

 six smaller tubercles. The outline is quite pentagonal, the lower sur- 

 face concave, but little undulating, the anus placed near the edge, and 

 covered, as in all preceding stages, by one plate ; the anal system in 

 older specimens has five plates, the plate first formed remaining some- 

 what the largest. As the young Echinarachnius increases in size its out- 

 line becomes more circular, and in specimens measuring one fifth of an 

 inch in diameter has the general appearance of the adult. The 

 furrows joining the ambulacral pores appear soon after the first traces 

 of a true rosette are seen ; they become deeper and the pores separate 

 in proportion with the petaloid structure of the abactinal part of the 

 ambulacrum. The tubercles are proportionally much smaller and more 

 numerous, and soon after the ambulacra have a well-developed rosette, 

 bear nearly the ratio to the plates which they have in the adult. 



