ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 799 



Obolus Maccullochi, sp. nov. 

 (Plate I, figs. 9-13.) 



1883. Lingula Ramsayi, Salter, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., p. 206, 

 pi. xvii, figs. 7-1 1 *? (only the specimens from Craighead). 



Shell broadly subovate, widest in front of middle, anteriorly rounded, as broad 

 as long or nearly so ; gently biconvex. Pedicle-valve acuminate ; beak pointed, 

 shoulders meeting at about 90°-100° and extending about one-third the length of 

 valve ; interior imperfectly known, but pair of small umbonal muscle-scars present 

 near beak, as well as pair of elongate impressed lateral muscle-scars diverging slightly 

 anteriorly and distinctly bounded at sides, apparently divided in front into unequal 

 halves longitudinally, and separated by impressed narrower median area (? visceral 

 cavity) towards front ; length and anterior ends of lateral muscles unknown. 

 Brachial valve shorter than pedicle-valve, with inconspicuous beak and obtusely 

 rounded posterior end ; interior of valve showing boundaries of splanchnocoel pro- 

 longed anteriorly into elongated median tongue nearly reaching front end of valve, 

 divided centrally by thin low longitudinal ridge; pair of oval "central" muscles 

 rather deeply sunk, situated at base of tongue, and pair of stout vascular trunks 

 diverging from base of splanchnoccel and continued forwards concentric to lateral 

 margins. Surface of shell ornamented with fine regular concentric closely placed 

 sublamellose lines of rather unequal strength ; interior of shell marked with fine 

 closely placed radial striae and a few pits posteriorly. 



Dimensions. — Length, 135 mm. ; width, 12'5 mm. 



Horizon. — Stinchar Limestone Group. 



Locality. — Craighead. 



Remarks. — The interior of the brachial valve which is exhibited in one specimen 



(118/III) in the Geological Survey Museum, Edinburgh, is of considerable interest, for 



it resembles that of Lingulella Sehvyni, Matthew, which Walcott # puts in the genus 



Obolus. Matthew had previously remarked that the plan of the muscular scars was 



very like that of Obolus apollinis Quenstedti (Mickwitz). The subgenus of Obolus 



termed Westonia by Walcott (op. cit., p. 450), ranges up into the Ordovician, and 



Lingulobolus , another subgenus, is confined to the Ordovician (Walcott, op. cit., 



p. 379). There is nothing, moreover, in the external characters against referring 



this Craighead shell to the genus Obolus as interpreted by Walcott, and the internal 



characters, so far as they are known, agree better with it than with Lingulella. In 



shape the shell much resembles 0. dolatus (Sardeson) t of the Ordovician of America. 



The internal radial striation is a common feature in species of Obolus. No good 



interior of a pedicle-valve is known to me. The more acuminate projecting beak of the 



pedicle-valve and the general linguloid shape and ornamentation of the shell, as well 



* Walcott, Camb. Brack., 1912, p. 413, pi. xxxvi, figs. 1, 2 ; pi. xxxvii, fig. 1. 

 Walcott, op. cit., p. 390, text-figs. 35a-c. 



