494 



FOSSIL BRACHIOPODA 



CHAP. 



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 living representatives 

 of these genera. In the most 

 highly specialised family of 

 the Ecardines — the T rimer el- 

 lidae — we meet with features 

 of peculiar interest.^ The 

 muscle-scars in this family 

 (Fig. 323, A, B) are most 

 remarkable for the develop- 

 ment of the so-called '' cres- 

 cent," {q.r.s.) which skirts 

 the posterior margin of both 

 valves as a sub-cardinal im- 

 pression. It is believed to 

 be the trace of a strong post- 

 parietal muscular wall, anal- 

 ogous in position to that of Lingula. The three pairs of 'lat- 

 eral" muscle-scars in the latter genus seem to be represented 

 by the "terminal" (s) and "lateral" (r) scars on the crescent 



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

 Inner surface of A, Pedicle-valve or ven- 

 tral valve. B, Brachial or dorsal valve; 

 p.s, parietal scar; u, umbonal muscle; t, 

 transmedians; c, centrals; a. m.e, laterals 

 (a, auteriors; m, middles; e, externals). 



1 The results of the investigations of King {Ann. Mag. Nat. Hist., 4th ser., 

 vol. xii., 1873) and of Brooks {Chesapeake 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 anato- 

 mists of the functions and homologies of the muscles. The lateral muscles enable 

 the valves to move backwards and forwards on each other; the centrals close the 

 shell; the umbonals 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. Geol. Sue, xxx. (1874), p. 124. 



