ORDOVICIAN BRACHIOPODA 55 



The exopuncta are shallow holes up to 0-2 mm deep, puncturing the external 

 surface of the shell at acute angles directed posteriorly. Within 3 mm of the um- 

 bones of each valve, the exopuncta tend to occupy the crests of costae. In larger 

 shells they alternate on either side of the crest line at intervals of about 0-5 mm, but 

 become more closely crowded and approximate to a double row in the marginal 

 zones of shells more than 7 mm long. Judging from the inferred arrangement of the 

 mantle canals relative to the costal embayments they probably accommodated 

 setae which dehisced at regular intervals as the shell grew forward to seal them off 

 from the mantle edge in the manner described by Rudwick (1965 : 604) for Acantho- 

 thiris. If this interpretation is correct the arrangement and incidence of the 

 exopuncta show that the growth of new setae not only balanced the loss of those 

 isolated by shell deposition, but also compensated for the increasing expansion of the 

 adult commissure as the decrease in forward growth tended to crowd the alternately 

 arising setae into double rows which appeared more or less simultaneously. This 

 pattern is different from that figured by Williams (in Williams et al. 1965 : H301) for 

 Orthambonites cf. rotundiformis Cooper in which the morphology of the shell margin 

 is compatible with the setal follicles having occupied narrow depressions in the 

 intercostal eminences and not in the costal grooves. Such depressions are vestigial 

 in 0. exopunctata ; but whether their presence is indicative of a more primitive 

 anatomy as is suggested by their occurrence in Arenigian Orthambonites such as 0. 

 calligramma Dalman remains to be demonstrated. 



Table 30 



Statistics of length (1) and maximum width (w) of n brachial valves of Orthambonites exopunctata 

 sp. nov. (A) and Lenorthis cf. proava (Salter) (B) 



A B 



n 17 20 



1 mm (var 1) 4*97 (5-877) 4-8 (2-804) 



w mm (var w) 6-74 (8-36) 6-84 (3-479) 



r 0-984 0-867 



logel (var log e l) 1-497(0-2132) 1-512(0-1145) 



log e w (var log e w) 1-8226 (0-1695) 1-886 (0-072) 



r e 0-985 0-873 



a (var a) 0-892 (0-0016) 0-793 (0-00829) 



Table 31 



Statistics of length (1) and depth (th) of n brachial valves of Orthambonites exopunctata sp. nov. 



(A) and Lenorthis cf. proava (Salter) (B) 





A 





B 



n 



13 





8 



1 mm (var 1) 



6-55 (5-389) 





5-84 (1-569) 



th mm (var th) 



1-18 (0-271) 





0-99 (0-059) 



r 



0-863 





0-617 



a (var a) 



0-224 (o-oou 



■7) 



0-193 (0-003 



