Several specimens from each situs were disjointed and a study of the individual valves showed that those from pholad-hole specimens were thicker and had shorter sutural plates and a wider sinus, this last being especially noticeable in the valves from the posterior end. Apparently this change in the sinus is the result of the broadening of the connecting ligaments due to compression by the crowding valves.
A count of the insertion plates of these disjointed specimens was made and considerable variation noticed. So much, in fact, that more specimens were pulled apart for the express purpose of counting these plates. Representative counts were as follows: 9 slits on the anterior valve, 2‒3 on the median valves, and 10 on the posterior. Others show 12, 2‒3, 8; 11, 2‒3; 14, 3‒4, 11. Absolutely no difference in this character could be found between specimens from the tidepools and those from the pholad holes.
On page 64 of vol. xiv of the Manual of Conchology, Dr. Pilsbry says, “Carpenter has given a varietal name to a broad, worn specimen which he thus describes:
“Var. _solidus_. Very solid, wide, ashen; inside whitish, the posterior valve with 10, central valve 2‒3, anterior valve 12 slits. Length 72, breadth 40, divergence 130°. Carpenteria, near Sta. Barbara, Cal. This is scarcely more than an individual variation. The mantle (girdle) is normal. The sculpture is worn away except at the edge. It has evidently lived in a very exposed situation.”
From this description and the figure which he gives of the valves it would seem that this is the same form as my specimens from the pholad holes. I fully agree with Dr. Pilsbry that it is hardly worthy of varietal rank. It is, however, too distinct in appearance to be labeled simply _Ischnochiton conspicuus_, Cpr., and I have therefore marked these specimens from the pholad holes _I. conspicuus_, Cpr., _form solida_, Cpr., using the term _form_ as advocated by Dr. Cockerell, “to designate variations plainly due to environment.”[1]
LAMPSILIS VENTRICOSA COHONGORONTA IN THE POTOMAC RIVER.
In 1912 Dr. A. E. Ortmann recorded[2] finding in the Potomac River a variety of _Lampsilis ventricosa_[3] to which he gave the name _cohongoronta_. His records were: