548 €. Lapworth — On Scottish Monograptidce. 



38. DiMORPHOGRAPTus SwANSTONi, sp. nov. Plate XX. Fig. ISa-c."^ 



Polypary less than one incli in length ; proximal portion mono- 

 prioniclian, narrow, recurved and having a blunt initial extremity 

 formed by a short stout sicula ; distal portion rapidly expanding to 

 a normal diameter of one-eighth of an inch. Hydrothecse 25 to 30 

 to the inch, without overlap ; making an angle of about 30° with 

 the general ventral margin ; outer edge of each concave ; apertural 

 edge deep and slightly oblique. 



The length of the proximal portion of the polypary varies from 

 one-eighth to nearly one-third of an inch, and the number of its 

 hydrothec^ from three to five. Except in its short and broad sicula, 

 this part of the fossil agrees precisely in all its characters with the 

 earlier portion of Monograptus gregarius, Lapw. The hydrothecse 

 are a little more prolonged in the diprionidian portion of the 

 polypary, and there is occasionally well-marked overlap in the 

 neighbourhood of the distal extremity. Here the apertures are 

 either directly visible, slightly foreshortened, or there is a broad 

 denticle, one margin of which is concave and the other convex. In 

 young examples appears a minute distal prolongation of the very 

 distinct filiform virgula ; in adult specimens it is wholly invisible. 



Zocality.— This well-marked little species has not yet been detected 

 in the South of Scotland ; but is here described and figured as strik- 

 ingly illustrative of this remarkable genus. Th^ specimens figured 

 were forwarded by Wm. Swanston, Esq., of Belfast, to whose cabinet 

 they belong, and who is at present engaged in working outthe Grapto- 

 litiferous bands of the North-East of Ireland with remarkable success. 

 In addition to the forms described in the foregoing jjages, there 

 •aTe several distinct South Scottish species of Ilonogi^aptidce of which 

 only fragments have hitherto been detected. Following out the 

 rule laid down at the commencement of this memoir, I shall defer 

 their description until both extremities of each form have been 

 recognized and its full diagnosis thus rendered possible. 



It may be that future investigation will show that some of the 

 forms here classed as varieties ought rather to be regarded as 

 distinct species. Even during the progress of the paper fragments 

 from the distal extreimtj oi llonograptus convolutus, His. ( = sp^Va/^s, 

 Geinitz), hitherto identified by its proximal extremity alone, have 

 been detected; and as the thecge are precisely similar in form 

 throughout the whole extent of the polypary, and thus strikingly 

 different from all those of the remainder of the Sedgwicki group, 

 there can no longer be any hesitation in regarding it as a distinct 

 species. It is very doubtful also if Monograptus dubius, Suess, can 

 long be retained as a variety of Monog. colonus. 



C)n the other hand, 31. Halli, Barr. ; M. Riccartonensis, Lapw. ; 

 and M. Flemingii, Salter, though so strikingly different in their 

 general aspect in the field that no one would hesitate to regard them 

 as distinct sj)ecies, yet approximate to each other so closely in the 

 form of the theca and its ornaments, that we may eventually be 

 forced to look upon them as being merely successive (chronological) 

 varieties of one and the same species. 



1 See Geol. Mag. for Nov. p. 507. 



