10 Proceedings of the Royal Physical Society. 



crassa (Hall), in the flagstones of Knockclolian and Arclwell, 

 a cliaracteristic species of the Utica slate of New York. 

 Again, a group of shells well marked in American Silurian 

 rocks are the Pleitrorhyncld, represented in the Craighead 

 limestone by the P. diptcrios (Salter), and peculiar to that 

 deposit. 



The univalve shells again indicate the singular coincidence 

 existing between many of the Girvan species and those of 

 the Silurian deposits of North America. We have, for instance, 

 Murchisonia angustata (Hall), in the Cuddystone conglomer- 

 ate, a fossil of the Bird's-eye limestone of the Trenton group. 

 By no means the least interesting of the organic remains 

 from Girvan is the curious genus Machtrca, with its abnormal 

 operculum or mouthpiece, represented by no less than three 

 species. These occur low in the series, in the Craighead, or 

 Aldons limestone, and at several localities. 



In America Maclurea is characteristic of the Lower 

 Silurian series almost entirely. It is supposed to have 

 belonged to that division of the MoUusca termed Hetero- 

 poda, or Nucleo-branchiata, in which the foot is modified to 

 act as a swimming organ, and in which the respiratory 

 organs are confined to a nucleus placed on the hinder 

 part of the back. Another genus of the same order well 

 represented in Girvan rocks is Bellero2:)hon, in which the 

 shell is symmetrical and convoluted, and the w^horls in 

 one plane. Salter determined four species in the Girvan 

 rocks. 



Lastly, those strange chambered shells, the OrthoccratiteSy 

 are represented by five or six species. They occur high in the 

 series in the Orthoceratite flags of Piedmont Glen, Ardwell, 

 etc., above the shelly sandstone of Mulloch. According to 

 M. Barrande the 0. ]_')olitum (M'Coy) from Piedmont Glen is 

 identical with a species from the lower part of the Bohemian 

 Upper Silurian. 



Contemporary with Salter's work on the Girvan fossils 

 were the researches of a brotlier pah-contologist, Mr Frederick 

 M'Coy, then assistant to the late Professor Sedgwick, now 

 Professor of Natur^il Science in the University of Melbourne. 

 Although not of the same extent, or depth of investigation 



