366 PROCEEDINGS OP THE ACADEMY OF 



from the la^t mentioned JState, kindly sent to me by Prof. Safford, the State 

 Geologist. The specific characters are as follows : 



Shell equilateral, or nearly so, very inequivalve. Lower valve deep, convex, 

 sometimes a little the longest at the lower left hand corner, when the valve 

 is laid on its face. Surface marked by six large radiating ribs, placed at about 

 equal distances, and with usually four, sometimes but three intermediate ribs. 

 When there are four, two or three of them are of about the same size, the other 

 one or two being much smaller and placed on the side of the adjoining princi- 

 pal rib. Between the most external principal rib, on each side and the ear, 

 there are from four to six fine linear ribs. The whole surface is crossed by 

 minute imbricating lines of growth. Upper valve flat or concave, and marked 

 by about from twenty-six to thirty nearly uniform, radiating ribs, with 

 wider concave interspaces. The tops of the ribs are subtriangular or regularly 

 rounded. There is, very rarely, the slightest approach to the sexradiate ar- 

 rangement of the other valve. I have not seen the ears, but Dr. Morton figures 

 them as being small and equal. He has them of nearly the right size, but I 

 think the left hand one, from the remains on my specimen, should be larger. 



Dr. Morton's figure is in the main correct. The basal margin should, how- 

 ever, be more excavated between the large ribs, and the intermediate ribs are 

 too numerous. The inequality of the two sides is well represented. 



This species can be distinguished from both N~. quadricostata and N. quinque- 

 costata by the upper valve. Instead of having six large ribs with three or four 

 smaller alternate ones, all of the ribs are of a nearly uniform size. The valves 

 are less undulate on their margins than either of the above species. 



Roemer calls this species a variety of "Pecten quadricostata." 



Ctenoides Klein. 



C. squarrosa n. s. Shell oblique, gibbons. Surface marked by about 

 ten or twelve larger uniform square ribs placed at about equal distances, nearly 

 fiat on top, and with semicircular concavities between them, a little wider thau 

 the ribs. The top of each rib is marked by two grooves, so as to make it tri- 

 costate. At the base of each interspace is a fine linear rib. The whole surface 

 is crossed by distinct lines of growth, somewhat inbricated. Ears unknown. 

 Height -6 in., greatest width -5 in., height of valve -18 in. 



The form of this species is about exactly like that of C. pelagicum, but it can 

 be distinguished by the much smaller number and relatively larger size of the 

 ribs. 



Locality and position. White cretaceous limestone of Alabama. 



One specimen. Coll. Academy. 



Teredo. 



T. t i b i a 1 i s Morton. Syn. p. 68, pi. 9, fig. 2. Dr. Morton includes two 

 very different fossils under this name. We will have to retain his name for 

 the species figured. The one to which he refers as occurring in the "friable 

 marls" is T. irregularis nob. T. tibialis, however, is not a Teredo, but is proba- 

 bly allied to Vcrmetus. It has never been found boring, but grows in aggrega- 

 ted masses of cylindrical tubes, almost always parallel and straight, sometimes 

 five inches long, slightly variable in diameter from irregular constrictions, con- 

 tains no shell, but the tube is divided at certain distances by transverse septa?, 

 convex and thin, the convexity pointing towards the widest (or newest) por- 

 tion of the tube, as if the animal progressed along the tubes, closing the space 

 behind it, as in the manner of the Cephalopoda, but hermeticall}'. I can find 

 no genus described, in which I can place this species, from the fact that the 

 shell is straight from the beginning. I therefore propose the generic name 

 Polorthus, and characterize it as follows : 



Shell tubular, straight or nearly so, growing in aggregated masses arising 



[Nov. 



