NATURAL SCIENCES OP PHILADELPHIA. 23 



very deep concave septa, and a somewhat elliptic section. Major angle of di- 

 vergence 5 in a specimen about four inches long, giving a taper of '09 in the 

 space of one inch ; ratio of axes of transverse section *9S : 1*32 = 1'34 ; ratio of 

 concavity of septa to greater diameter "38 : 1*2 = 3*16 ; annulations, ten in the 

 space of three inches, where the mean transverse axis is 1"27, making their 

 distance a little less than one-fourth this axis, the plane of the annulations 

 forming an angle of 74 with the longitudinal axis of the shell, the opening 

 generally toward the extremity of the shorter diameter ; the annulations hav- 

 ing a broad, shallow, retral sinuosity, which in some specimens is distinctly 

 marked, and in others obscure ; position of the sinuosity somewhat variable ; 

 space between the annulations regularly concave ; a peculiar obsolescence of 

 the annuli sometimes occurs along the side marked by the sinuosity, and not 

 unfrequently a partial or nearly complete obsolescence of entire annuli takes 

 place along a portion of the shell. Siphon large, situated on the longer di- 

 ameter two-sevenths of the distance from the centre to the periphery. Sur- 

 face (of cast) marked by feeble, rounded, encircling striae, which generally run 

 parallel with the annulations, but sometimes cross them at a small angle. 



The deep concavity of the septa is a strong distinctive mark of this, amongst 

 annulated species. In many specimens this is much deeper than in the large 

 specimen from which the measurements have been taken. The occasional 

 obsolescence of the annulations may also be noted. 



Orthocekas heterocixctum, n. sp. Shell unequally annulated. rather 

 rapidly tapering, with a nearly circular section and a subcentral siphon. Angle 

 of divergence 11 ; annulations inequidistant, strong, rounded, separated by 

 concave intervals, somewhat oblique and slightly sinuated retrally on one 

 of the sides, very often becoming more or less obsolete, and sometimes en- 

 tirely disappearing ; septa slightly oblique ; siphon on the longer diameter 

 less than its own diameter distant from the longitudinal axis of the shell. 

 Surface marked by unequal striae running parallel with the rings. 



Resembles 0. Whitei in the occasional obsolescence of the rings ; and casual 

 observation would not distinguish the two. The present species, however, 

 tapers more rapidly, is less compressed, has a less excentric siphon and exhibits 

 a much more frequent disappearance of the annuli. 



Ranges from the base of the yellow sandstones into the base of the Burling- 

 ton limestone. 



Orthoceras Indianense, Hall, (Twelfth Rep. N. Y. Reg., p. 10.) Numerous 

 specimens of an Orthoceras having a circular or slightly eccentric section, a 

 central or subcentral siphon and an angle of divergence of about 8 fail to 

 furnish any characters for distinction from the above species. If such identi- 

 fication is correct, this species enjoyed considerable geographical range. 



Ranges from the bottom of the yellow sandstones into the base of the Bur- 

 lington limestone, in company with crinoids regarded as characteristic of the 

 latter formation. 



PHRAGMOCERAS, Broderip. 



Pheagmocebas expansum, n. sp. Shell slightly arcuate in the earlier stages 

 of growth, becoming nearly straight at a later period ; somewhat gradually 

 enlarging at first, but afterwards expanding with great rapidity, finally under- 

 going a gradually reduced rate of expansion, which, at the aperture of the 

 adult, amounts to a slight constriction ; transverse section very slightly com- 

 pressed laterally ; position of siphon unknown ; septa transverse, moderately 

 concave. Surface of cast smooth. 



In a specimen which is -7 1 - 1 in diameter at the small (imperfect) end, the 

 same diameter increases to 2-1 in the space of 1-37 inches, showing an angle, 

 of divergence of about 68 c . 



CYRTOCERAS, Goldfuss. 

 Cyrtoceeas ujticobne, n. sp. Shell arcuate ; angle of divergence when young 

 1863.] 



