MTCROPLAX. 21 



This shell, externally, resembles some of the coarse, ill-defined 

 Acanthochitons. The girdle, however, is narrower and smoother 

 than in that genus, and the pores so extremely minute that in a dry 

 specimen they would escape attention. Within, however, the 

 features are entirely new though not unexpected. It forms a tran- 

 sition between Hanleya (mendicaria) and the articulate chitons. A 

 single unslit lamina surrounds both the terminal valves, projecting 

 far beyond the external layer. In the posterior plate this is con- 

 tinued forward to form part of the sutural laminae. These in all 

 the valves slope off both toward the middle and toward the sides, so 

 as to take the place of the ordinary side-laminae, which here do not 

 exist. 



Genus IV. MICKOPLAX Adams & Angas, 1864. 



Microplax H. AD. & ANG., Proc. Zool. Soc. 1864, p. 194, type M. 

 grayi Ad. & Ang. CARPENTER MS., p. 12. 



Insertion plates smooth and thin, present in all the valves. Sut- 

 ural plates obsolete, the sinus extremely shallow. Girdle thin, 

 horny, most minutely granulous. Valves largely concealed in the 

 girdle, the exposed portions small and separated. 



In the present genus a small portion only of each valve is exposed ; 

 and the sutural plates and sinus are obsolete. No other Chiton 

 having unslit insertion plates approaches this remarkable group. 



M. GRAYI Adams and Angas. PI. 6, figs. 9, 10, 11. 



Shell elongated, convex, brown ; exposed portion of the valves 

 minute, wide heart-shaped, carinated, strongly granulated, the inter- 

 vals between the exposed parts of the valves about as long as the 

 latter. Lateral areas defined by a distinct rib. Girdle moderate, 

 corneous, smooth (A. and A~). Length 13, width 5 mill. 



Sydney Harbor, Australia, under stones at low water. 



Microplax grayi H. AD. and ANG., P. Z. S. 1864, p. 194, /. c. 

 1865, p. 58, t. 11, f. 16. ANGAS, P. Z. S. 1867, p. 224. CARPEN- 

 TER MS., p. 12. 



The figures given were drawn by Mr. E. A. Smith from the type 

 specimen. 



Carpenter's notes upon the unique specimen in coll. H. Adams 

 are as follows: It is quite transparent, looking like thin horn. 

 There are very slight overlappings of the valves, corresponding to 



