MATTHEW] STUDIES ON CAMBRIAN FAUNAS 119 



border the surface of the shield is somewhat depressed, and behind the de- 

 pression are two slight elevations. The anterior end projects forward and 

 is sharpl)^ rounded. From this end the lower border runs with a regular 

 curve to the posterior border, whence to the fold or hinge-line the border 

 becomes straighter. There appears to be the broken base of a spine at 

 this posterior angle. From the anterior to the posterior angle a marginal 

 groove runs along the lower border of the shield. 



Sculpture. — The surface of valve is granulated, but shows traces of 

 grooving concentric to the anterior end of the hinge line. There is a row 

 of shallow pits in the hollow behind the cardinal fold, consequent on these 

 growth-lines or grooving». 



Size. — Length, 3^ mm. ; width at anterior end of the hinge-line 2 mm. 

 Eare. 



mut. ARCUATA (PL YII., fig. 6). 

 A. terranovicus mut. arcuata, Nat. Hist. Soc. N.B., Bull. xviii.,p. 195, pi. ill., fig. 6. 



A variety of the above, or another species, has a shorter and more 

 tumid valve, and a rounder curve at the anterior end. The hollow between 

 the main swelling of the valve and the axial fold is crossed by four ribs 

 parallel to the posterior border ; the anterior of these folds is the pos- 

 terior border of a low crescentic area which curves downwai-d and forward 

 toward the anterior end of the valve ; a low ridge extends from the 

 anterior end of the axial fold across this area to the middle of the pos- 

 terior border. 



Sculpture. — The surface of the shield is granulated. 



Size. — Length, 3 mm. ; width, 1|- mm. Eare. 



In conclusion I may add that my warm thanks are due to Mr. J. V. 

 Howley for information in reference to Smith Sound and its geology, as 

 well as for permission to use his section of the north shore of that sound. 



And my cordial acknowledgments are also due to Mr. Gilbert van 

 Ingen, the curator of the museum in Columbia University, New York, 

 for copies of the original figures and description of Palœacmœa typica, 

 upon which the genus Pala^acma^a was founded, and which is related to 

 a genus I have found to be present in this ancient fauna. I wish also to 

 acknowledge my obligation to Dr. E. T. Jackson, of the Museum of Com- 

 parative Zoology, Cambridge, Mass., in having procured for me a copy of 

 Shaler and Foerste's article on the Cambrian Fauna of Attleboro', Mass. 



