THE FAUNA OF THK ST. JOHN GEOUP. 127 



larval forms of all the genera. The later stage of growth, in which a narrow, thread-like 

 rim appears at the front of the prominent and overhanging glabella, is more nearly akin 

 to the adult form in Ellipsocephalns. Another variation in the form of the anterior 

 margin is that wherein a channel or depression is formed within the margin of the shield, 

 with or without a marginal fold. This occurs in Solenopleura, but is better exemplified 

 in the several varieties of a species of Liostracus. One variety shows a marginal channel, 

 smooth, and hollow to the very edge, with all the strife (raised lines) of growth outside 

 the rim ; while in others, the striœ appear on the raised fold of the upper margin ; similarly, 

 striae may be observed ouside of the raised edge on the free cheeks of the genus, but 

 none within. It is not improbable that in the first stages of all the trilobites of this 

 family, the folded rim is deficient, and that the stiifening of the margin, which is in most 

 cases produced by the inward folding of the rim, is in Liostracus obtained by a reverse 

 bending of the margin, and in Agraulos by the bowl-shaped rounded front of the cheeks. 

 As the closely-folded margin is the more prevalent among the later examples of this 

 family in the Lower Cambrian, it may be regarded as the more advanced expression of 

 growth in this part ; hence Agraulos and Liostracus are to be regarded as retaining more 

 primitive outlines in this part than the other genera, except Ellipsocephalns. 



Occipital Ring. — There is considerable diversity in the form of the ring in the genera 

 of this group, and in considering its value for classification, based on development in the 

 young, it is well to consider its expression in the earlier forms of the families described in 

 previous papers. Paradoxides, as seen in the young of P. Eleminicm, has an occipital ring 

 which approaches a triangular form ; but in the adult it become lenticular. The same 

 change may be observed as marking the growth of Conocoryphe, but is not so clearly 

 shown in Ctenocephalus, because that genus (as exhibited in the Acadian area) retains a 

 pointed ring in the adult. A triangular neck-ring is present in several of the genera to 

 which this paper relates ; and in those which do not possess it in the adult stage, as in 

 Solenopleura, it is sufficiently marked in the young. Agraulos is the genus which above 

 all the others is characterised by a heavy triangular neck-segment, and Liostracus comes 

 next it. An important consideration in relation to the development of the neck-ring is that 

 in the earliest larval form, it is scarcely or not at all marked oflf from the rest of the glabella, 

 a feature which, it will be seen, is retained by the adult test in Ellipsocephalns. Soleno- 

 pleura may be regarded as more advanced than the others, because of its short neck-ring, 

 devoid of a spine behind. One species (a Solenopleura?) had a spine at the back of the 

 neck-ring, which fell away after the casting of the test (or death of the animal). This spine 

 had a crescent like base, and occurs loose with other fragments of these trilobites. 



Movable Cheek. — The change in the form of this part of the head-shield is largely comple- 

 mentary to the variation of the dorsal suture and eyelobe, and the development of the 

 anterior marginal fold ; but in one respect, this is not the case. The youngest tests of this 

 family, in the forms known to me, have no genal spine, but the spine is developed in 

 the adults of some species. The movable cheek of some species of Ellipsocephalns and 

 Agrairlos,' of the St. John group have not been recognised. In Liostracus there is no 

 spine, but behind the back part of the cheek, which is stiffened by a ridge on the marginal 

 fold, is a short, flattened lappet, which bends in behind the posterior margin of the head- 



' A narrow cheek, bearing a spine, which may belong to this genus, occurs in the shales of Ic. 



