278 BULLETIN OF THE NATURAL HISTORY SOCIETY. 
on the front part, the radiating ridges. No cicatrix marking the 
advance of the foramen was observed, but a progressive change of 
this kind is probably indicated by the paired thread-like ridges behind 
the foramen on the interior of the ventral valve. 
Size . — Length and width each 4 mm. Depth about § of a milli- 
metre, that of the dorsal valve less. 
Horizon and locality . — Fine dark grey shales of the Dictyonema 
beds (C. 3c) at McLeod Brook, Cape Breton. 
This pretty little species is the smallest and oldest known of its 
genus. Mr. Walcott indicates for S. typicalis a calcareo-corneous shel^ 
but while there may be an outer calcareous layer to S. prisons, it has 
not been detected.* From the former species which is Ordovician, it 
differs not only in its small size, but its orbicular form ; it differs alsc 
in having radiating as well as concentric striae on the outer surface 
It is much smaller than Dr. Ami’s S. canadensis of the Utica shale 
In one example of the ventral in this species the foramen is in the 
umbo, but in the others it is in front of it. The ring around the 
inside of the foraminal opening is never prolonged into a tube as in 
Siphonotreta. 
In re-examining the material from this horizon at Navy Island, 
St. John, I find that this species is present there also, but the surface 
markings are not well preserved ; however the form and size of the 
shell, and the foraminal opening, show it to be the same species. 
Agnostus trisectus, Salt, mut. ponepunctus, n. mut. PI, V, figs. 8«-c. 
This form grows to a larger size than the type as figured by Tull- 
berg, and differs in several respects. The reticulation on the head 
shield does not show a net-work near the glabella, but detached 
irregular furrows ; opposite the posterior half of the glabella the 
ornamentation is scarcely more than small, sparse, irregular pits. 
The posterior end of the glabella is wider than that of the European 
form, and there are lateral lobes on the front of the main lobe. 
In the pygidium there are also differences; the sculpturing of the 
side-lobes is scarcely more than shallow, open pits, faintly visible, and 
there is a small tubercle at the end of the rachis, which overhangs the 
rachial furrow. Examples of the pygidium showing the inner surface 
have as many as nine paired pits along the inner furrow of posterio 
* U. S. Geol. Surv., Monog. viii, p 70, pi, i, figs. 3, 3 a to c. 
