320 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Fig. 8. 
the fossil and not to the left as in testudinalis. Professor C. Fred Hartt 
described a shell from the Cambrian of St. John Basin as a brachiopod 
under the name of Discina acadica. Dr. G. F'. INIatthew (S), with 
better material, described the structural features of the shell and says 
that Mr. R. P. Whitfield first drew attention to the calcareous nature 
of the shell and suggested that it was a gastropod allied to Palaeacmaea 
or Metoptoma. Dr. C. D. Walcott afterwards placed it in the genus 
Stenotheca. Matthew gives a figure of the 
shell from the interior showing the char- 
, , acteristic horseshoe-shaped muscle scar 
I / \ opening at the anterior end while the apex 
of the shell is directed backward. He says 
that it lived in shallow seas along the coast 
judging from the species associated with it, 
and was probably a bottom-crawler. The 
shell he figures is about four millimeters in 
length and the apex is one millimeter from 
the posterior margin. In the above compar- 
ison I have assumed that the nucleus or 
apex of Stenotheca is posterior as it is in 
Acmaea ^^in its early stages. Palaeontologists in recognizing Scenella, 
Stenotheca, Tryblidium, and others as related to Acmaea and its 
allies have assumed that the position or direction of the beak or nucleus 
indicated the anterior region as it does in recent Acmaea and Patella. 
To determine the anterior margin of these Cambrian fossils, it would be 
necessary to examine the muscle scar within. This horseshoe-shaped 
scar opens over the head of the creature and consequently indicates 
the anterior margin. Matthew (8) figures the interior of one of these 
forms from the Cambrian, and the muscle impression is clearly shown. 
Its opening indicates the anterior region. The apex of the shell is 
nearer the opposite margin which is therefore posterior as it is in the 
early stages of Acmaea of the present day as herein described. In the 
living Acmaea studied the a])ex is about central when the shell is about 
one millimeter long and by the time it reaches two millimeters in 
length the apex is within half a millimeter of the anterior margin and 
this position is retained till maturity. As we have shown, in the ex- 
treme young the apex is at the extreme posterior margin of the shell. 
In these early Cambrian forms delicate lines are seen radiating from 
the apex to the rim of the shell. Dr. Walcott has kindly sent me 
