THE FAUNA OF THE ST. JOHN GROUP. 153 



supposed to be modified in the narrow form, that this would seem to be its actvial relation. 

 The specimens in the Hartt collection figured by Mr. Walcott are very imperfect, and 

 probably do not fairly represent the position of the eyelobe, which, except in very young 

 tests, is further back. 



Our examples of S. Orestes shew it to have a deeper dorsal furrow than S. Robbii, and 

 the glabella, as Prof. Hartt has said, is more distinctly ridged on the crest, and it is a fifth 

 longer than wide, while in S. Robbii the length and width are equal ; the glabellar 

 furrows are less arched and the posterior goes more directly backward, both the glabella 

 and the occipital ring are more compressed than in S. Robbii and the posterior marginal 

 furrow is wider, the posterior marginal fold is angulated upward in the middle. 



Movable Cheek (Plate II, figs. 4c and d). — This is scarcely as wide as that of the broad 

 form and is bluntly terminated, or in some examples \veakly pointed. 



Thorax. — A young example of this form in the Museum of the University of New 

 Brunswick (Plate II, fig. 4a) exhibits well the peculiarities of this form. There are 

 fourteen joints in the thorax. The rachis is about half of the width of the pleural lobe. 

 The pleurae are weakly geniculated, in the anterior segments more than half way from 

 the rachis and at the tenth segment just half way from it ; the segments behind the tenth 

 rapidly shorten. The pleurae are traversed by a sharply impressed furrow which extends 

 nearly to the extremity ; the extremities of the pleurae are bluntly rounded. 



Pygidivm (Plate II, fig. 4e). — It is broadly lenticular, having a prominent rachis con- 

 sisting of four rings : of these, the first two are distinctly elevated, but the furrow dividing 

 the two posterior rings is faint, so that they sometimes seem to form one lobe ; at the pos- 

 terior end of this lobe, the projecting extremities of the last ring sometimes have the 

 appearance of a pair of obscure tubercles ; behind these rings, the rachis rapidly descends 

 to an obscure obtuse point, nearly reaching the end of the pygidium. The lateral lobes 

 of the pygidium are traversed by two main furrows, of which the anterior is more deeply 

 impressed, and gives origin at its inner end to a diverging faintly marked furrow ; beside 

 these two furrows, there is near the rachis the rudiment of a third furrow, diverging 

 backward. 



The surface markings on both the outside and inside of the test are much the same in 

 the two forms. 



Length, about 26 mm. Width, about 19 mm. Length of head shield, 10 mm. Width 

 between the sutures, 15 mm. Length of pygidium, 2J mm. ; width, *7 mm. 



Horizon and Locality. — In the grey shales of Div. 1 c, at Portland, Simouds and St 

 Martin's. At Portland the narrow form (P. Orestes) is more plentiful than the wide. 



The head-shield of this species (S. Robbii), as Mr. Walcott has observed, resembles 

 that of S. cristuta, Linrs., but it differs in having a deeper dorsal furrow and in being 

 acuminate at the aenal an^le. 



o^ 



Development of the Young. 



The shields of the young of this species cannot be distinguished from those of the 

 species of Liostracus, which occur with it, when of a less length than J ram. : the 

 prominent cylindrical glabellar ridge abruptly terminated in front, the short depressed 

 anterior margin and the nearly straight suture, are features common to the minute shields 



