THE FAUNA OF THE ST. JOHN GROUP. 141 



Except for the great prominence of the rachis iu this organism, we should not be able 

 to distinguish even the family of the trilobite by the form of its test : for, with its single 

 ring iu the pygidial part, and the enlarged anterior end of the rachis in the cephalic 

 portion of the embryo, the later stages might be those of a Conocorypheau as readily as 

 of a Ptychoparian trilobite. Possibly we may see iu the obscure occipital ring of this 

 little embryo an autetype of the faintly marked ring of the adult iu Ellipsocephak\s. No 

 eyes are visible at this stage of growth. 



^nd Stage. — Iu the next stage (PI. I, fig. 4 m), in which the cephalic shield and 

 pygidium have been separated, features appear iu the former which enable us without 

 difficulty to distinguish the trilobites of this family from the Conocoryphinoe ; these 

 features are the less perfectly semicircular form of the head-shield, the greater promi- 

 nence and comparatively larger size of the glabellar ridge, and the presence of eyelobes. 

 These, in the examples known, are at first very prominent, and no ocular fillet has been 

 observed ; if present, it must be out of sight from above the shield, but the eyelobe itself 

 stands prominently outward on the lateral margin. At this stage may be observed the 

 faint, but clearly defined, threadlike line of the folded anterior margin for a short space 

 in front of the glabella, but there is no trace of a facial suture. 



3rd Stage. — A slight advance beyond this stage is exhibited by PI. I, fig. 4 n. Iu 

 this the form is much the same as in the last, bvit the ocular fillet has now become visible, 

 and the eyelobe no longer stands out so prominently from the edge of the shield. The 

 anterior marginal fold also is more distinct than in the preceding form. The ocular fillet 

 at this stage is close to the anterior marginal fold, owing to the very slight development 

 of the dividing furrow ; in this it resembles the young of the Conocorypheau s, and, as in 

 that family the ocular fillet recedes from the front of the shield as the glabella shortens 

 during the growth of the animal. 



In the advanced position of the ocular fillet, as well as in the long and prominent 

 glabellar ridge in these little head-shields, we are again reminded of Ellipsocephalus ; 

 but a counterpart to the straight marginal suture, and the short eyelobes comparatively 

 distant from the anterior margin, is to be found in Agraulos rather than iu Ellipsoce- 

 phalus. The form of the anterior margin in these young tests, exhibits affinities in 

 another direction ; the front is suddenly and strongly depressed, and is bounded by a 

 narrow, threadlike, sharply-upturned luarginal fold ; this is characteristic of the genus to 

 which these small tests belong, biit it is also, iu a greater or less degree, characteristic of 

 Soleuopleura, and more markedly of subgenus Strenuella in Agraulos. 



4th and bth Stages. — In the smaller stages of this species there is a considerable 

 resemblance to the corresponding stages of Agraixlos as will be seen by a comparison of 

 the figures in the two first columns on Plate I. Liostracus, however, is distinguishable 

 by its shorter and broader glabella and shallower dorsal suture. Figs. 4 n and o of 

 Plate I, which are nearly equal in size, but are different stages in the development of 

 the species, exhibit wide differences of form, and shew that at corresponding stages of 

 dcA'elopment, the embryonic (and larval) forms differed considerably in size, or else that 

 the change from the embryonic to the larval form was very rapid. ' 



The later stages of this trilobite present gradual modifications of the test until the 



' Compare also the corresponding stages in Agravios Hallianus, PI. i, figs. 2 h and i. 



