154 Mr. C. Lapwortli on new British Graptolites. 



slight convexity of the proximal extremity is precisely similar 

 to that in the adult portions. The width of the entire poly- 

 pary, except near its initial extremity, is constant ; but the 

 length of the slowly tapering portion is occasionally as much 

 as three or four inches. 



In specimens preserved with their full relief the hydrothecse 

 are seen to be a little wider at their point of origin than near 

 their outer orifice ; and the general aspect of the polypary 

 under this aspect reminds us somewhat of those of the group 

 typified by M. colonus, Barr. In casts and compressed ex- 

 amples the distal portion of the theca appears as a wide perpen- 

 dicular tube, lying parallel to the axis of the polypary, and 

 occupying about one third of its total diameter. In these speci- 

 mens the apertural margin is deeply concave, is very slightly 

 oblique, and shows a rudimentary denticle. 



This species is allied, on the one hand, to Monograptus 

 leptotheca, Lapw., and, on the other, to M. galaensis, Lapvv. 

 It differs from the former in the shape and amount of overlap 

 of the hydrothecae. In the proximal portion of M. crenularis 

 the hydrotheca? are twice as numerous for the same distance 

 as in that of M. leptotheca ; in the adult portion of the latter 

 species they are more than twice as long as in the species 

 under description. From M. galaensis the present form is 

 separated by the form of the distal portion of the hydrotheca, 

 which in the former species is strongly denticulate. 



Horizon and Locality. Birkhill Shales. Zone of Cephalo- 

 graptus cometa at Dobbs Linn, Crosscleuch, &c. 



In the Upper Llandovery, Wenlock, and Lower Ludlow 

 formations of Britain occur at least four doubtfully distinct, 

 closely allied forms of Monograptus^ which seem to repre- 

 sent the Bohemian species Monograptus Halli of Barrande. 

 They all agree in the general shape of the hydrotheca, which 

 is tubular, of subequal diameter throughout the whole of its 

 extent, with a more or less expanded and everted aperture, 

 the outer margin of which is broadly convex and frequently 

 spinose. They differ from each other mainly in the size of 

 the polypary, the number and amount of overlap of the hydro- 

 theca?, &c. They are all easily distinguished from the type 

 form of Barrande's M. Halli (see PI. IV. fig. 9 a) by the 

 inclination and form of the hydrotheca?. The first of these 

 British forms seems to be peculiar to the Llandovery ; the 

 second and third are Wenlock forms, while the fourth 

 doubtfully transgresses the limits of the Lower Ludlow. 

 Their individual peculiarities are given in the following- 

 diagnoses. 



