POLYPLACOPHORA. 



Vll 



and the latter corresponding closely in sculpture to the lateral areas* 

 In some genera the mucro is near or at the posterior edge of the 

 valve, and the posterior area is then reduced to a narrow tract, or 

 altogether absent (see pi. 52, figs. 17, 18). In position, the mucro 

 may be either anterior (pi. 30, fig. 23), or median (pi. 17, fig. 22) or 

 posterior (pi. 52, figs. 22, 23) ; and it may be either elevated (pi. 36, 

 fig. 94), or depressed (pi. 39, fig. 41), the last being sometimes called 

 a flat or planate mucro. 



THE INNER LAYER OF THE VALVES (articulamentum) is larger 

 than the tegmentum, projecting in front in two lobes called sutural 

 lamince, which are separated by a median bay, ihejugal sinus. At 



the sides of the intermediate valves, 

 and around the semicircle of the end 

 1 valves, most Chitons have projecting 

 ^ plates called insertion plates, to which 

 a the girdle is attached. These are 

 * commonly cut into teeth by transverse 

 slits. From the slits to the apex of 

 each valve inside, run slight grooves 

 and rows of pores, known as slit-rays; 

 often they are obliterated, but in forms 

 having a highly developed system of 

 sense-organs in the tegmentum, the 



Figs. 3, 4. Intermediate and P 3reS f the slit ' ra y s Se rve as nerve 

 posterior valves of hchnochiton foramina. The teeth are sometimes 

 conspicuus. finely cut or crenulated, (technically 



" pectinated ") between the slits ; and in some forms the edges of the 

 teeth are thickened outside, or " propped." Fig. 3 represents the 

 interior of the fourth valve of an Ischnochiton, showing the slits, 

 teeth, insertion plates, etc. 



The tegmentum is essentially cuticular in nature. It consists of 

 a chitinous foundation substance, impregnated with salts of lime. 

 It is perforated at the surface by a multitude of minute, definitely 

 Arranged pores of two sizes, called by Moseley, the larger megalopores, 

 the smaller micropores (see pi. 52, fig. 28, showing two megalopores 

 and many micropores). Each megalopore leads into a cylindrical 

 chamber which is continued below into a wide canal, which as it 

 penetrates deeper toward the plane of junction of tegmentum and 

 articulamentum, curves outward toward the girdle-margin of the 

 former. On reaching the plane of junction it joins a plexus of wide 



