32 THE NAUTILUS. 
to the comparatively motionless portion at the sides of each valve, 
resulting in a more exuberant growth of girdle spicules there. 
Within the Acanthochites stock the progressive diminution of the 
tegmentum or outer layer of shell, has proceeded along two lines: 
in one series of forms the girdle has encroached at the sutures, pro- 
ducing a heart-shaped exposed area, seen in such species as the Noto- 
places, and this system has also produced the Amiculas. In the others, 
the tendepey has been to encroach along the sides of the valves, leay- 
ing a narrow or linear tract, resulting in forms like Acanthochites 
exguisitus, and culminating in Cryptoconchus (C. monticularis Q., 
and floridanus Dall.) 
Acanthochites is divisible into four sections: Acanthochites typi- 
cal, having a wide caudal sinus and two slits in the tail valve, and 
well-developed sutural tufts; Notoplax, having several slits in the 
tail-valve behind, and the girdle encroaching at the sutures; Oryp- 
toconchus, having a similar tail-valve, but the girdle encroaches at 
the sides, leaving only a linear dorsal area exposed ; and finally, 
Loboplax (sect. nov.), with a many-slit tail-valve, the head valve 
strongly 5-lobed and ribbed, girdle nearly naked—type A. violaceus 
Quoy. The following two species belong to the typical section : 
A. exquisitusn.sp. Visible portions of the valves extremely narrow, 
generally less than one-fourth the entire width of the dried animal. 
Valves dark olive, interior blue; the girdle light green, tufts very 
large, either green, pink or bronze; fleshy covered with a green 
pubescence. Length 30, breadth 18 mill. La Paz (Lockington). 
The valves are more covered than in any other form, the tegmen- 
tum being far less in area than one of the sutural amine. 
A. rhodeus n. sp. Exposed portion of valves subtriangular, about 
one-third the entire width, the valves depressed, obtusely carinated, 
brown, almost separated by the encroachment of the girdle at the 
sutures. Median area smooth, not striated. Interior deep rose 
colored. Length 28, breadth 15 mill. Panama (MacNeil). 
A. (Notoplax) hemphilli n. sp. Valves heart-shaped, about one- 
third the total width; red, more or less maculated with white; 
girdle rust-brown; dorsal area having some longitudinal striz. 
Interior light green at the sides, deep rose-red in the middle. Girdle 
wide, sparsely clothed with microscopic hyaline spicules, having a 
marginal row of longer spicules and 18 small white tufts. Length 
24, breadth 11 mill. Key West, Florida (Hemphill). 
