J. HOPKINSON ON BRITISH GRAPTOLITES. 155 



linos, which gradually approximate towards the aperture (PI. viii., 

 f. 2, 4, 5, and \'2). The curvilinear hydrothecaj are united only 

 at their base ; while the tubular have their superior or distal margin 

 entirely in contact with the lower portion of the inferior or proxi- 

 mal margin of the next hydrotheca (PL viii.,/. 3, 6, 7, 9 (?), 10, 

 11, 13, and 14^. The rectangular differ only in being angular in 

 section, instead of round (^PZ. viii., /. 8, 18, and 20). Sometimes 

 they are in contact for their whole length (PI. viii.,/. 18 and 20). 

 In the incised forms the inferior wall of each hydrotheca forms the 

 superior wall of the next (PL viii., /. 15. 16, 17, and 19/ I have 

 already alluded to them as having no distinct hydrothecse. 



The inferior margin of the aperture of the tubular hydrotheca is 

 sometimes ornamented with a mucronate extension, or spine (PL viii., 

 /. Zb). In the incised, the superior margin of the aperture is some- 

 times extended into a mucronate point (PL viii.,/. 19/ In some 

 species there is a row of minute pustules near the base of the 

 hydrothecjB {PL viii.,/. lib). In many fine strife, parallel to the 

 margin of the apertures, are apparent (PL viii.,/ 13^^ ; they ap- 

 pear to be lines of growth. The periderm of some genera is ii 

 regularly striated ; in others, it has a granular appearance. In one 

 or two species, I have detected, where the cuticular layer, or ecto- 

 derm, appears to have been removed — a reticulated appearance. But 

 from the imperfect state in which graptolites are preserved, the 

 nature or utility of these surface markings is doubtful. Careful 

 study of well preserved specimens under the microscope,* and 

 especial care in the illumination, may clear up these and other 

 doubtful points of structure. 



§ II. History. 



I can only touch briefly on the history of the British graptolites. 



Bromel most probably alluded to graptolites, when, in describing 

 the fossils of Sweden, in 1727, he speaks of the fossil leaves of 

 grasses. 



Linnaeus, in the first edition of his " Systema Naturae " (1736), 

 applies the name Gixvptolithus to certain natural objects, fucoid 

 markings, worm tracks, &c. In his " Scanian Travels " (1751), he 

 first describes and illustrates under this generic name, a trvie grap- 



* A three or four-inch object glass should be used. A four-inch, made for 

 me by Mr. Collins, shows structure much better than a higher power. 



