CLASS III AMPHINEURA 511 



numerous, occupying the groove between foot and girdle ; radula present, heterodont ; 

 sexes separate. 



The external covering in the Polyplacophora, or Chitons, consists of eight 

 valves bound together by an encircling flexible band called the girdle. The 

 anterior or head-plate (Fig. 841, ^-^j below) is invariably semicircular, with the 

 apex or mucro at the middle of the straight margin ; the six succeeding plates 

 are generally square (Fig. 842, below), with the apex posterior on the median 

 line ; and the posterior or tail- valve (Fig. 841, ^) is semicircular or subcircular, 

 with apex varying in position from in front of the middle to the hind margin. 

 All of the plates are composed of two layers — an outer porous layer, the 

 tegmentum, and an inner porcellanous one, the articulamentum. In most of the 

 lower Chitons these layers are coextensive and have smooth edges ; but in 

 the higher forms the articulamentum projects beyond the outer layer into the 

 substance of the girdle, in Avhich it is firmly inserted. These projections at 

 the outer or peripheral margin are termed insertion plates. They are generally 

 slit or notched into so-called " teeth," which may be either smooth and sharp 

 along the edge, or crenulated {pectinated). Insertion plates serve the function 

 of binding the valves firmly to the girdle. 



The anterior margin of each valve except the first one invariably shows 

 two projections of the articulamentum called sutural laminae (Figs. 841, B, 

 842), which pass under the rear margin of the next anterior valve, thus pre- 

 venting vertical displacement of the series. The tegmentum is traversed by 

 a multitude of fine canals which terminate at the surface in minute sense 

 organs. The cavities of the latter in dry or fossil valves are visible as fine 

 quincuncial punctations. In the highest Chitons a certain number of these 

 sense organs have become enlarged and modified into eyes, easily recognised 

 as pigmented dots in recent, and small pits in fossil specimens. 



Polyplacophora make their appearance as early as the Ordovician ; they are 

 rare in the Silurian and Devonian, but somewhat more abundant in the 

 Carboniferous. None of the Paleozoic genera is known to continue into the 

 Mesozoic, but the Eoplacophora are replaced by types more related to modern 

 Chitons {Mesoplacophora). Members of the most specialised suborder, Teleo- 

 placophora, first appear in the Eocene, although they doubtless arose earlier. 

 About twenty Paleozoic, five or six Mesozoic, and fifty Tertiary species have 

 been described. Recent forms number several hundreds. A good many 

 species supposed to be Chitons have been based upon barnacle valves, fish 

 scales, and other fragments. The recently described Duslia insignis Jahn is 

 apparently a Crustacean ; certainly not a member of the Polyplacophora. 



Three suborders are recognised, according as the insertion plates are 

 absent, or if present, unslit (Eoplacophora) ; developed, smooth, and slit into 

 teeth (Mesoplacophora) ; or both slit and pectinated (Teleoplacophora). 



Suborder A. EOPLACOPHORA Pilsbry. 



Polyplacophora with the tegmentum coextensive with the articulamentum, or with the 

 latter projecting in smooth, unslit insertion plates ; gills posterior. 



Family 1. Gryphochitonidae PiLsbry. 



Insertion plates absent, sutural laminae small ; one or both end-valves with the 

 terminal margins elevated; form elongated and narroiv. Paleozoic. 



