4 



AMPHINEURA. 



[Polyplacophora. 



each side, the dorsal or jiagal area remaining distinct. This modi- 

 fication is characteristic of one great phylum of Chitons. As a rule, 

 the head valve is sculptured all over like the lateral areas. The 

 posterior valve (IV) is divided into a central and a posterior area, the 

 former being precisely similar to the area so named in the intermediate 

 valves, 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. In position, the mucro may be either anterior, 

 or median, or posterior ; and it may be either elevated or depressed, 

 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 laminae, 

 which are separated by a median bay, the jugal sinus. At the sides of 

 the intermediate valves, and around the semicircle of the end valves, 

 most Chitons have projecting plates called insertion plates, to which 



I. INTERMEDIATE VALVE OF ISCHNOCHITON. 

 1-3, central aares (1, jxigal tract ; 2, 3, pleural tracts) ; 4, 5, lateral areas ; 



6, sutural laminae. 



II. INTERMEDIATE VALVE OF ACANTHOCHITES. 

 7, insertion plates ; 8, dorsal area ; 9, latero- pleural areas. 



III. INTERMEDIATE VALVE. 

 10, sinus; 11, valve-callus; 12, slits ; 13, teeth ; 14, slit-rays ; 15, eaves. 



IV. POSTERIOR VALVE. 

 16, central area ; 17, posterior area ; 18, mucro. 



the girdle is attached. These are commonly cut into teeth by trans- 

 verse 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 pores of the slit-rays serve as nerve- 

 foramina. The teeth are sometimes finely cut or crenulated (technic- 

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

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

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

 teeth, insertion plates, &c. 



