ANNULOSA I CRUSTACEA. 239 



fessors Dana and Verrill, however, regard these so-called legs 

 as being " the semicalcified arches in the membrane of the 

 ventral surface, to which the foliaceous appendages or legs were 

 attached." The specimen in question was an Asaphus ; but 

 the great number and excellent preservation of Trilobites, as a 

 general rule, render it highly probable that in most cases the 

 limbs were destitute of a chitinous exoskeleton, and were there- 

 fore incapable of being preserved in a fossil state. According 

 to Spence Bate, "the young of the Trilobites are of the 

 Nauplius form." 



The cephalic shield of a typical Trilobite is more or less completely 

 semicircular (fig. 89, 2), and is composed of a central and of two lateral 

 pieces, of which the two latter may, or may not, be united together in 

 front of the former. 



The median portion is usually elevated above the remainder of the 

 cephalic shield, and is called the "glabella;" it protected the region of 

 the stomach, and is usually divided into from three to four lobes by lateral 

 grooves. At each side of the glabella, and continuous with it, is a small 

 semicircular area, called the " fixed cheek." The glabella, with the " fixed 

 cheeks," is separated from the lateral portions of the cephalic shield 

 termed the "movable" or "free cheeks" by a peculiar suture or line 

 of division, which is known as the "facial suture," and is quite unknown 

 amongst recent Crustacea, except for a faint indication in the Limttlus, and 

 more or less doubtful traces in certain other forms. The movable cheeks 

 bear the eyes, which are generally crescentic or reniform in shape, are 

 rarely pedunculated (being never supported upon movable foot-stalks), 

 and consist of an aggregation of facets covered by a thin cornea. The 

 facial sutures may join one another in front of the glabella in which case 

 the free cheeks will form a single piece or they may cut the anterior margin 

 of the shield separately in which case the free cheeks will be discontinu- 

 ous. The posterior angles of the free cheeks are often produced into long 

 spines. 



Behind the cephalic shield comes the thorax, composed of a variable 

 number of segments, which are not soldered together, but are capable of 

 free motion upon one another, so as to allow the animal, in many cases, to 

 roll itself up after the manner of a wood-louse, or hedgehog. The thorax 

 is usually strongly trilobed, and each thorax-ring shows the same trilobation, 

 being composed of a central, more or less strongly convex, portion, called 

 the " axis," and of two flatter side-lobes, called the " pleurae." 



The " pygidium," or " tail," is usually trilobed also, and, like the thorax, 

 consists of a median axis and of a marginal limb, the composition of the 

 whole out of anchylosed segments being shown by the existence of axial 

 and pleural grooves. 



ORDER IV. MEROSTOMATA. The members of this order 

 are Crustacea, often of gigantic size, in which the mouth is 

 furnished with mandibles and maxillae, the terminations of 

 which become walking or swimming feet, and organs of pre- 

 hension. 



This order comprises the recent King Crabs, and the extinct 

 Pterygoti and Eurypteri. 



