350 NEW YORK STATE MUSEUM 



fourth spines. My figure shows that considerable variation occurs even 

 in a single specimen. There are two pairs of incisions, the first between 

 the first and second lobes and the second outside the latter lobe. The 

 first incisions are usually very deep, while both pairs are narrow. 



The ends of the -lobes are heavily chitinized. The chitinous processes 

 extending along the mesal margins of the median lobes I have not found 

 large and definite so frequently as in A. forbesi. Those on the inner 

 margin of the first incision are of the general type of the latter species 

 and are strongly curved toward the lobe, but are more usually broader at 

 the base of the process, that is, toward the tip and outer edge of the lobe. 

 This is not, however, a very positive difference. The opposite processes 

 nearly equal these in size, which is not the case in A. forbesi. Some- 

 times I have not distinguished the process of the outer edge of the second 

 incision, but it is often present of about the same size as that of the 

 inner edge. 



Ventral glands are wanting even in the adult. The chitinous thick- 

 enings of this region are present however as definite and narrow dark 

 areas in well cleared specimens. They are twice bent and of the shape 

 shown in the figure. These will serve to differentiate the adult A. p e r- 

 n i-c i o s u s from the immature forms of any of these species, even though 

 eggs and young are not present to certify to the maturity of the form. 



The anus is of medium size, and is about as far from the ends of the 

 chitinous processes as these are from the tips of the lobes. 



The transverse chitinous processes are frequently broad and some- 

 what irregular. They are not so usually prominent and definite as in 

 A. ostreaeformis. The dorsal pores are comparatively lew, small 

 and inconspicuous. They are usually present in traces of three short 

 lines; the first runs from the second incision; the second just laterad of 

 this and the third, consisting of only a few scattering pores, is still farther 

 laterad. Quite frequently a single pore is to be seen anterior to the 

 lateral transverse thickening. 



The second stage (pi. 12, fig. 2) is much the same in general arrange- 

 ment and in the outfine of parts except that the ventral chitinous thicken- 

 ings are parenthesis-shaped and lack definiteness, the dorsal pores are 

 fewer and more scattering, and the plates are not always so easily detected. 

 It may be most easily confounded in this stage with A. forbesi, but 

 it usually displays plates enough in carefully prepared specimens to sepa- 

 rate it from that form, and will show also, on greater familiarity, the same 

 difference in relative size of the chitinous processes and in relative posi- 

 tion of the lobes which is to be marked in the adult forms. 



