348 NEW YORK STATE MUSEUM 



differ from the allied species. The plates when present are few and 

 small; occasionally one or two may be detected between or beside the 

 lobes. 



The chitinous processes are very characteristic, though hard to 

 describe. The tips of the lobes are quite heavily chitinized, and the proc- 

 esses extending cephalad along the inner margins of the median lobes 

 are usually comparatively large and distinct. Newell, in his Iowa bul- 

 letin no. 43, p. i6i, speaks of these as sharp pointed and curved in con- 

 tradistinction to the almost straight ones of A. ancylus. This 

 difference, if constant, takes very careful focusing to determine, and is 

 not striking, as is proven by the fact that other illustrators of the species 

 have not brought it out. Indeed, the original figure represents these 

 processes as straight and quite different in shape from NewelPs figure. 

 This point is probably good additional evidence of the identity of the 

 species when it can be definitely ascertained, but is not the most 

 obvious and easiest criterion. There is also an extension of chitin 

 cephalad near the outer margin of the median lobes. 



The chitinous processes on the inner margin of the fir§t incision are 

 large, much larger than those opposite. They are almost pear-shaped 

 with a compound curve on the side toward the median line. That is, 

 they are often abruptly narrowed toward the tip and outer margin of the 

 lobe with a full curve to the very broad and plump cephalic part. This 

 may seem a hazy description till somewhat cleared by study of the 

 figures and by familiarity with the forms. As a matter of fact these 

 chitinous processes are quite distinctive in A. f o r b es i being easily dis- 

 tinguished from the more indefinite and irregular ones of A. ostreae- 

 f o r m i s and A. ancylus and somewhat less certainly from A. p e r - 

 n i c i o s us by the narrower distal part, and specially by the usually marked 

 superiority in size to the opposite process, while in A. perniciosus 

 the two processes are subequal. The chitinous processes about the 

 small second incision vary more than in the case of the first incision. 



The adult female shows ventral glands in 5 groups. " Johnson gives 

 1-3 for the anterior group, 3-7 for the anterior laterals and 3-5 for the 

 posterior laterals. The general arrangement of these is somewhat linear. 



The ventral chitinous thickenings within the posterior lateral groups 

 are nearly straight and club-shaped and are usually narrow and definite, 

 as stated above. Sometimes a fainter, less heavily chitinized fold or fork 

 appears, but the main and obvious process is more or less nearly as illus- 

 trated and is usually in line with a small, straight, more heavily chitinized 

 portion of the general thickening inward from the base of the lobes men- 

 tioned in the paragraph for the explanation of terms. 



