140 G. F. MATTHEW : ILLUSTRATIONS OF 



glabella with diverging furrows, this form is comparable with those of the genus 

 Solenopleura. 



Var. GIBBA (PI. I, figs. 4 /; to /.-.) 



This is a small form with inflated head-shield ; it has an oval glabella, as the last, a 

 downward slope in front of the glabella and cheeks, and a strong occipital spine. 



Horizon and LocalUy. — Div. 1 c ' (" Calcareous conglomerate "), St. Martin's. 



This form, which is an older variety than any of the preceding, approaches Agraulos 

 in its oval glabella, depressed front, inconspicuous eyelobes and strong occipital spine. 



Var. PLANA (PI. I, figs. 4 e to g) 



This small form accompanies the preceding, but is distinguished by its wider and 

 flatter head-shield, its more prominent eyelobes, and by the coarser sculpture of its surface. 



Horizon and Localitij. — Same as the preceding variety. 



This bears a general resemblance to the typical form of the species, but difters in its 

 larger and more oval glabella and its heavier occipital ring and spine. 



Development of the Young (PI. I, figs. 4 Z to q) 



Among the head-shields of the smaller trilobites with eyes, most frequently met with 

 in the shales of Baud c, the remains of this species and of Solenopleura Robbii are most 

 common. One may, therefore, describe in this connection a small oval body, which is 

 ■ evidently the first stage in the development of a species of the family Ptychoparidte. 

 There is a still more rudimentary object which is occasionally met with in these shales, 

 but which being a minute globular body with an umbilical depression, may represent the 

 egg of any trilobite ; but the oval body above referred to, may, with some probability, be 

 considered as belonging to the family now under review. 



1st Stage. — In tracing, down to the smallest discoverable individuals, several species 

 of Paradoxidese, which occur in the St. John Cambrian basin, a tendency to expansion 

 rather than contraction of the anterior margin has been observed ; these minute indivi- 

 duals also have a very attenuated glabella ; a comparatively weak and narrow glabellar 

 axis also marks the minute head of the Conocoryphintc of these shales. Hence it is 

 supposed that the small oval body, represented at PL I, fig. 4 I, is the earliest stage of a 

 Ptychoparian trilobite, as the glabellar ridge is of greater prominence and size than in the 

 embryos of the two subfamilies named. 



The rachis in this form is distinctly raised above the rest of the test throughout its 

 whole length, and the cephalic shield and pygidium are indicated by a strong groove 

 across the axis, about two-thirds of its whole length from the front of the test ; at this 

 stage, the occipital ring is faintly outlined by a shallow groove across the rachis, and a 

 similar but fainter groove indicates the position of the first ring of the axis of the pygi- 

 dium. The cheeks of the cephalic shield and the side lobes of the pygidium still form a 

 continuous rounded surface, except that in some examples these lobes are divided by a 

 faint transverse line, indicating the formation of a suture between the elements of the 

 head-shield and the pygidium. 



