142 Hall on the 



Genus Graptolithus (Linn.). 



Description. — Coralluui or bryozoum fixed, (free ?) compound 

 or simple ? the parts bi-laterally arranged, consisting of few or 

 many simple or variously bifurcating branches, radiating more or 

 less regularly from a centre, and united towards their base in a 

 continuous thin corneous membrane or disk, formed by an expan- 

 sion of the substance of the branches, and which in tbe living state 

 may have been in some degree gelatinous. Branches with a 

 single or double series of cellules or serratures, communicating with 

 a common longitudinal canal, affixed by a slender radix or pedi- 

 cle from the centre of the exterior side. 



The fragments, either simple or variously branched, hitherto 

 described as species of Graptolithus, are for the most part to be 

 regarded as detached portions from the entire frond. 



In its living state we may suppose it to have been concavo-con- 

 vex (the upper being the concave side), or to have had the power 

 to assume this form at will. In many specimens there is no evi- 

 dence of a radix or point of attachment, and they have very much 

 the appearance of bodies which may have floated free in the 

 ocean. 



Graptolithus Logani. 



Plate I. Fig. 1—6. Plate II. Pig. 1, 2, 3, 4. 



Description. — Frond composed of numerous branches nearly 

 equally disposed on two sides of a central connecting stipe, and 

 each again subdividing nearly equally, after which they bifurcate, 

 always near the base, with greater or less regularity ; connecting 

 membrane thin, composed of the same substance, and continuous 

 with the branches, and extending from the centre to some distance 

 beyond the bifurcations ; the branches after the third bifurca- 

 tion become marked on the inner side by a row of cellules, and 

 along the centre by an abruptly depressed line which follows the 

 divarication of the branches ; cellules minute, not prominent to- 

 wards the base of the branches, being compressed vertically, and 

 appearing like a double series with a central depressed line, becom- 

 ing developed as they recede from the base.- The branches 

 beyond the disk are turned on one side and laterally flattened, 

 and present a single series of cellules or serrations, which are 

 moderately deep, with the serratures acute at their extremities ; 

 from twenty-four to twenty-eight in an inch. The substance of 



