BRACHIOPODA. 21 



umbonal muscle. No Lingula possesses this peculiar cx'esentic ridj^e, but it is a 

 feature strongly developed and of great importance in the various genera con- 

 stituting the Family TitntiKELinKE, of Davidson and King, and in this respect, 

 as well as in others to be directly noticed, Lingulops is a Trimerellid. 



In the arrangement of the internal impressions on the pedicle-valve, within 

 the crescent, there is a general similarity to Lingula. An elongate median 

 impression, evidently the progressive scar of the muscles homologous with the 

 middle laterals of Lingula, is accompanied, on either side, by scars of the central 

 muscles, and outside of these lie faint scars (much better defined in L. Nor- 

 woodi than in L Whitfieldi), which may be ascribed to the outside laterals.* 



These median scars have a sharply elevated anterior edge, but otherwise 

 their surface is depressed in the older species, L. Whitfieldi and L. Norwoodi, 

 while, in L. Granti, the muscular area is thickened over its entire surface, form- 

 ing an incipient platform. From the anterior edge of this area, extends a 

 median septum, sometimes of considerable length. This, in L. Whitfieldi, is 

 accompanied, on either side, by a deep groove, outside of which lies a broad 

 and low rounded ridge. 



In Lingula, the median septum does not attain such a development in the pedicle- 

 valve, except in forms referable to Dignomia, where it extends forward from 

 the posterior extremity without interruption. In the brachial valvef of Lingula, 



*In regard to ihe !i(iin('nc;latui-e of the musculiip and other impressions in the platform-bearing' inartic- 

 ulates, it seems best to adopt that proposed by the English authors, though it is purely ai-biti-ary and con- 

 veys no correct impression of the functional relation of these parts to those of Lingula. In the pedicle-valve 

 of Trimerblla, the anterior and median scars cori-espOnd in position to the middles (k) of Ling ula ; the laterals 

 to the centrals (It), and outsiders, or externals (?) ; the terjiiinal scars of the crescent to the anterlors (J) and 

 transmedians {i) ; in the brachial-valve, the anter'wrs and medians, to the anteriors (j ) ; the laterals to the 

 centrals {?/), and the crescent scars to the middles (k) and transniediatis (i). LmarLOPS shows a very clearly 

 defined transition stage between these genera in muscular charactei'S, and though it has been impossible to 

 demonstrate, from observation, the existence of the transmedian muscles in the Trimerelloids, as no speci- 

 men has been seen which indicates that the lateral scars of the crescent are unsymmetrical, the same fact is 

 true of fossil Lingulas generally, and in default of other evidence, we are left to infer that the muscular 

 functions in all these forms were essentially similai-. 



t Messrs. Davipson and King were strongly convinced that the original specimen of L. Whitfieldi 

 should be regarded as the brachial rather than the pedicle-valve. Their opinions, which are entitled to the 

 most resiiectful consideration, are essentially these : The slit upon the cardinal area, which we have termed 

 the pedicle-groove, was very faintly defined in the imju'essions at their disposal, and they regarded it as a 

 depression, " nothing more than such as occasionally occurs in the brachial valve of Lingula anatina " (p. 

 166). Furthermore: "In the pedicle- valve of Trimerella the crown of the crescent has a forward curve 



