190 MAZATLAN UNIVALVES 
In the following descriptions, as far as practicable, the terms 
are employed as proposed by Middendorff, Mal. Ross. p. 36. 
By the jugum is meant the ridge running along the middle of 
the valves; the mwero is the apex on the posterior valve: the 
valve-lobes are the prolongations of one valve which fit under 
the next; the sinus lies between them. 
Genus LOPHYRUS, Poli. 
HT. § A. Ad. Gen. vol. i. p. 469.—Radsia+ Gymnoplax, Gray.— 
Chiton, pars, avet.—Plates of insertion in denticulated lobes ; 
integument tessellated with smooth scales. 
248. LopHyRuUs ARTICULATUS, Sow. 
Chiton articulatus, Sow. in Proc. Zool. Soc. March, 1832, 
p. 59 :—Coneh. Ill. no. 29, f. 18.—Zool. Beech. Voy. p. 150, 
pl. 41, f. 16. 
Lophyrus articulatus, H. & A. Ad. Gren. i. 470. 
The figure in the Conch. Til. is very accurate ; but that in 
Beech. Voy. represents much too strong a sculpture. The 
Mazatlan specimens rarely display any sculpture at all, being 
almost universally eroded, even in young specimens. The 
plainness of the exterior is however abundantly compensated 
for by the great beauty of the inner structure of the valves, 
which the large number of speeimens sent allow to be freely 
examined. The form is sometimes broad, with nearly straight 
sutures ; sometimes elongated, with arched back, and sutures 
bent, occasionally into the form of a —_—.. Very rarely 
the shell is indented on each side of the jugum, rudely present- 
ing a likeness to a Trilobite. The surface of the valves, when 
perfect, is crowded with minute pustules; diagonal lines 
scarcely marked. Colour olive green shaded into reddish 
brown at the ridge, with irregular longitudinal streaks of 
brown olive on each side: faint narrow rays of the same tinge 
on the terminal valves, and irregularly waved penciling over 
the surface of the diagonal areas. Inside bluish green; valve- 
lobes long and flattened, with a broad medial sinus. The 
surface within, under the microscope, is extremely finely cor- 
rugated. The margin is formed of fine, irregular plates, each 
sometimes branching into a plume; these are interrupted in 
the medial valves by one slit on each side, in the terminal ones 
by 14-20. From these proceed rows of punctures to the axis 
