190 MAZATLAN UNIVALVES 



In the following descriptions, as far as practicable, the terms 

 are employed as proposed by Middendorff, Mai. Ross. p. 36. 

 By ihejugum is meant the ridge running along the middle of 

 the valves ; the mucro 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 LOPHYBUS, Poli. 



H. fy A. Ad. Gen. vol. i. p. 469. Radsia + Gymnoplax, Gray, 

 Chiton, pars, auct. Plates of insertion in denticulated lobes ; 

 integument tessellated with smooth scales. 



1 248. LOPHYBUS AETICULATUS, Sow. 



Chiton articulatus, Sow. in Proc. ZooL Soc. March, 1832, 

 p. 59 -Conch. III. no. 29, f. 18. Zool. Beech. Voy. p. 150, 

 pi. 41, f. 16. 

 Lophyrus articulatus, H. fy A. Ad. Gen. i. 470. 



The figure in the Conch. III. is very accurate ; but that in 

 Seech. 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 specimens 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 



