41 



Incerta sedis. I believe Thamnograptus doveri is the only species 

 known in the Skiddaw Slates. 



These groups are further divided into genera, and the genera 

 into species ; but in the limited time allotted to me I shall only be 

 able to describe two or three of the species in each genus. 



MONOPRIONIDIAN GROUP. 



Azygograptus. — I believe there is only one species of this genus 

 known, namely, A. lapworthi (Fig. 8). It consists of a simple 

 monoprionidian polypary or stipe, which attains its full width close 

 to the proximal end. The stipe is usually long and slender, and 

 the portion occupied by the common body larger than in many of 

 the monoprionidian forms. The cellules are broad, numbering 

 about twenty in the space of an inch. Azygograpti occur very 

 plentifully at Hodgson Howe Quarry, near Portinscale, and some 

 specimens have been found at Barf. The rock in which the 

 fossils are found at Hodgson Howe is of a hard gritty nature, 

 occurring in a series of thin beds interstratified with soft slate. 

 The latter does not appear to have been suitable for the preser- 

 vation of the Azygograpti, as I believe that none have been found 

 in it hitherto ; but the gritty beds contain large quantities, often 

 matted together like tufts of grass. 



The genus Didymograptus comprises forms that consist of two 

 monoprionidian stipes springing from an initial point, which is 

 frequently marked by a distinct spine, called the "radicle." This 

 initial point, although it forms the base of the organism, is some- 

 times so small as to be scarcely discernible. The cellules usually 

 occupy the side of the stipe opposite the radicle, but in some cases 

 they are on the same side. D. tiitidus (Fig. 9) is the most 

 common species of this genus, indeed, they have been so numerous 

 in some localities, that stones are occasionally found covered with 

 a matted network of their fossilized remains. The two stipes 

 spring horizontally from the radicle, they are narrow at the base, 

 and widen very gradually towards the extremities, which are often 

 somewhat rounded, owing apparently to the imperfect development 

 of two or three of the last cellules. The widest parts of the stipes 



