494 



FOSSIL BRACHIOPODA 



Internal Characters 



Owing to the rarity of well-preserved interiors of valves 

 in this division, our knowledge of their internal characters is still 

 far from satisfactory. The arrangement of the muscular impres- 

 sions varies greatly amongst extinct genera, but we are often able 

 to interpret them with a considerable amount of certainty by a 

 study of the scars and the muscles of the well-known recent 

 Lingula (Fig. 322). The extreme specialisation of the muscles in 

 many of the earliest genera (e.g. Lingula) is remarkable, and points 

 to a long but so far undiscovered ancestry in pre-Cambrian times.^ 

 In fossil species of Crania and Lingula the muscle-scars correspond 



closely with those in the liv- 

 ing representatives of these 

 genera. In the most highly 

 specialised family of the Ecar- 

 dines — -the Trimerellidae — 

 we meet with features of 

 peculiar interest.- The muscle- 

 scars in this family (Fig. 

 323, A, B) are most remark- 

 al)le for the development (jf 

 the so-called " crescent," (r/.r.5.) 

 wliich skirts the posterior 

 margin of both valves, as a 

 sub -cardinal impression. It 

 is believed to be the trace 

 of a strong post - parietal 

 muscular wall, analogous in 

 position to that of Lingida. The three pairs of " lateral " 

 muscle -scars in the latter genus seem to be represented by 

 the " terminal " (s) and " lateral " (?■) scars on the crescent 



Fig. 322. — Muscle-scars of Lingula anatina. 

 Inner surface of A, Pedicle-valve or ventral 

 valve. B, Brachial or dorsal valve ; ^>.6', 

 parietal scar ; (^ umboual muscle ; t, trans- 

 mediaiis ; c, centrals ; a.m.e, laterals («, 

 auterions ; vi, middles ; e, externals). 



^ The results of the investigations of King {Ann. May. Nat. Hist. 4th ser. 

 vol. xii. 1873) and of Brooks {Clicsapcakc Zool. Laboratory, Scientific Results, p. 35, 

 1879), and the simple nomenclature of these authors are here followed in preference 

 to those of others, owing to the difference of opinion amongst anatomists of the 

 functions and liomologies of the muscles. The lateral muscles enable the valves to 

 move backwards and forwards on each other ; the centrals close the shell ; the um- 

 bonals open it ; and the transmedians allow a sliding sideways movement of one 

 valve across the other (see also p. 477). 



- Davidson and King, Quart. Jour. Gcol. Sue. xxx. (1874), p. 124. 



