EARLY DEVONIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA 121 



curved to the anterior margin which is transverse. Ventral valve gently 

 and quite uniformly convex, somewhat depressed to the cardinal angles. 

 Cardinal area carrying a row of spines, five in number on each side of the 

 beak, the outer ones attaining considerable length. Surface markings con- 

 sisting of fine threadlike radii increasing rapidly by bifurcation, the striae 

 and intervening grooves being of subequal size. There are three or four of 

 these in i mm. A notable feature is the predominant size of the median 

 stria on this valve. There are also suggestions of concentric or oblique 

 undulation near the cardinal extremities. The surface sometimes shows a 

 broad undefined depression with others at the side which may produce a 

 gently undulated surface. This, however, is not a persistent feature. The 

 dorsal valve is concave and on the interior shows a small bifurcate cardinal 

 process flush with the cardinal area. The sockets and socket walls rest on 

 a greatly thickened ridge just within the hinge line and subparallel to it. 

 This notable ridge has an abrupt posterior slope leading down to the 

 muscular area which is divided by three short and divergent ridges. 



Dimensions. The average example has a length of 16 mm, width of 

 23 mm. 



In seeking comparison of this very well defined species with allied 

 forms we may note the following : 



With Chonetes canadensis Billings of the Grande Greve fauna, 

 it is more closely related than with any other, in outline and proportions. 

 Like that it carries a conspicuous median stria. But the species are not to 

 be confounded ; C. aroostookensis is a stouter and heavier shell with 

 a much coarser surface striation and a more convex ventral valve. It is 

 less delicate and tenuous and never attains the notable dimensions of that 

 species. With C. nova-scoticus Hall from the Arisaig series of 

 Nova Scotia, it agrees in the development of the median stria but the 

 resemblance there ceases. Chonetes latus v. Buch as identified by 

 Sowerby from the Tilestones of Horeb Chapel, with which it has been com- 

 pared, has not even remote relation with it. Davidson long ago pointed 

 out that most of the Silurian Chonetes which had been referred to 

 C. latus are identical with C. striatellus Dalman but he specially 

 excepted the forms from Horeb Chapel. Neither the one nor the other 

 presents any features for comparison here, the Tilestones shell being small, 

 convex and minutely striate. C. sarcinulatus Schloth., from the Spiri- 

 ferensandstein and other horizons of the Coblentzian is somewhat similar 

 in form but is more evenly striate, without large median stria and is notably 

 convex. Schnur's variety of this species, pi anus, from the same beds is 

 little known but appears to be a shell of less width. 



Of all the species of early Devonic age, C. falklandicus, Morris 

 and Sharpe 1 presents the closest similarity though of smaller size and 



1 Quar. Jour. Geol. Soc. 1846. 2 : 274, pi. 10, fig 4. 



