LINGULA.. 199 



Family— LINGULIDJE. 



Genua — Lingula, Bruguiere 178!'. 



The limit of variation among the shells composing the genus Lingula appears to be 

 more restricted than what is prevalent among the generality of other genera and species of 

 Brachiopoda. It is, therefore, very often no easy matter to distinguish and correctlv 

 determine some fossil species, even when occurring in different and often widely separate 

 geological periods. 



The shell of Lingula is thin, equilateral, usually longer than wide, and broader at the 

 front than at the beaks, which are likewise more or less pointed, while the front is either nearly 

 straight or with a slight inward or outward curve. The shell is also sub-equivalve ; the 

 extremity of the beak of the dorsal valve being somewhat more elongated and pointed than 

 that of the ventral one. 1 The external surface is also either nearly smooth or concentri- 

 cally striated. The valves are usually moderately convex, and generally deepest or most 

 elevated towards the beak, and become more flattened as they approach the front. The 

 apex of the dorsal valve is likewise situated quite close to, but not contiguous with, the 

 rounded margin of the beak, and by which character the valves can be readily distinguished 

 both in the recent and fossil condition. 



When alive, the valves of Lingula were slightly gaping at each end, contiguous onlv 

 along the lateral margins ; but the animal could, at its will, by the action of certain 

 muscles, close or draw together one or other extremity ; nor does there exist any articu- 

 lation, the valves being kept in place by the means of a complicated system of muscles, to 

 be hereafter described. The animal was also provided with a very long pedicle, of a 

 peculiar construction, which was chiefly attached to the inner groove situated in the beak 

 of the ventral valve ; and when alive, did not inhabit great depths, most recent species 

 having been found at low water, buried in sand. 



The intimate shell-structure of Lingula has been described by Dr. Carpenter, and we 

 will therefore only refer to Dr. Gratiolet and Mr. S. Cloez's more recent observations. 

 The first-named savant states that the shell is composed of two distinct elements, the one- 

 being horny, the other shelly. That they are disposed in layers, or thin lamina3, which 

 succeed each other alternately from the convex surface of the valves, the outer or superficial 

 one being horny ; that these layers have not the same thickness, the testaceous ones being 

 the thickest on and near the visceral side, while the horny ones are more so towards the 

 exterior surface ; and that while the horny layers are entirely formed of parallel fibres, 



1 Anatomists appear to differ as to the names by which the valves should be designated ; it may 

 therefore be as well to mention those that are synonyms. The shortest is the dorsal valve of ^Yood\vard. 

 Hancock, &c. ; = valve inferieure, Gratiolet ; = valve droit e, Vogt. The longest is the ventral valve of 

 Woodward, Hancock, &c. ; = valve superienre, Gratiolet ; = valve gauche, Vogt. Mr. Hancock is of opinion 

 that if the names of the valves were to be changed, that they should be called anterior and posterior. In 

 this monograph we will continue to make use of those first mentioned. 



