Vol. XI] BERRY— FOSSIL CHITONS 423 



portion, technically known as the inner o, is usually evident as a 

 submedian, conical projection of greater or less prominence, 

 and is a characteristic feature. 



The shelly substance in all the valves is characteristically 

 formed of two layers, — an inner, usually hard, semi-porcel- 

 lanous tissue known as the articulamentum, and an outer, 

 usually softer, very different appearing, surface layer, the teg- 

 mentum. The articulamentum generally projects past the teg- 

 mentum on the sides and in front to form the mechanism of 

 articulation, namely the sutural laminae already described, and 

 the toothed or simple insertion plates which serve for the at- 

 tachment of the girdle at the sides of the median valves and 

 around the ends, anterior or posterior as the case may be, of the 

 terminal valves. The insertion plates are usually divided by 

 one or more slits into more or less evident teeth. Sometimes 

 evident in the articulamentum are thread-like lines, or lines of 

 pores, running radially from the valve umbo and leading- one 

 into each slit, thus marking the position of the slit through the 

 previous stages of growth. The sutural laminae are separated 

 in the central line by a sort of bay leading back nearly or quite 

 to the margin of the tegmentum, the sutural or jugal sinus. 



Those portions of the tegmentum just over where the in- 

 sertion plates push from under are referred to as the eaves, 

 and the tissue of the little cliffs so formed as cave tissue. This 

 tissue sometimes evinces special structural features of value. 



Dorsally, the tegmentum is typically capable of delimitation 

 into several distinct areas, which, in spite of modifications in 

 a considerable number of species, yet maintain themselves 

 through the group with a somewhat remarkable constancy. In 

 the median valves the regions of the tegmentum lying back of 

 the radial articulamental lines above mentioned, and thus over- 

 lying and coinciding with the old insertion plate tissue, are 

 generally a little elevated, or may be bounded by a ridge or 

 line of sculpturing in front. Their sculpture is subject to elab- 

 oration on a different plan from that of the rest of the valve 

 and is primarily radial, rather than longitudinal as elsewhere. 

 They are apt to be very definite in their boundaries and are 

 known as the lateral areas. The area lying in front of them 

 is called the central area. The region adjacent to the ridge or 

 jugum of the valve, except, of course, in the head valve, is known 

 as the jugal tract and the more lateral portions occupying the 



