BRACK 10 POD A. 21 



umbonal muscle. No Lingula possesses this peculiar cresentic ridge, but it is a 

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

 stituting the Family TniMKitEi.i.n>.K, 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, oxi either side, by scars of the central 

 muscles, and outside of these lie faint scars (niucli 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, inL. 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, 



*Iii legaid to ihe nomenclature of the muscular anil other impressions in the j)latfurm-l)eaiin{f iiiai'lic- 

 ulates, it seems best to adopt that proposed l)y the English authors, though it is jiiirely arbitrary and con- 

 veys no correct impression of the function.al relation of these parts to those of LiNtiL'LA. In the pedicle-valve 

 of Trimerella, the (interior and media?!, scai-s coi-respond in po.sition to the middles (k) of LiXdrtA ; the laterals 

 to the centrals (//), and outsiders, ov crternals (?) ; the terminal scars of the crescent to the anteriors (J) and 

 transmfdians (i) ; in the Lrachial-valve, the anteriors and medians, to the anterior.i (j ) ; the laterals to the 

 centrals (A), and the crescent scars to the middles (k) and transmedians (i). Lixgtlop.s shows a very clearly 

 defined transition stage between these genera in muscular chai-acters, and though it has been inipo.<sible 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 :ire left to infer that the nniscular 

 functions in all these forms were essentially similar. 



t Messrs. Daviosox and Ki.vii 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 respectful consideration, are essentially these : The slit upon the cardinal area, which we have termed 

 the pedicle-gi-oove, was vei'y faintly defined in the imjn-essions at their disposal, and they regarded it as a 

 depre.ssion, " nothing more than such as occasionally occui's in the brachial valve of Lingula anatina " (p. 

 168). Furthermore: "In the pedicle- valve of Tkimereli.a the crown of the crescent has a foriwrd curve 



