PALAEONTOLOGY OF THE UPPER MISSOURI. 



MOLLUSCA. 

 CLASS BRACHIOPODA. 



FAMILY LINGULID^E. (See page 1.) 

 Genus LINGULA, BRUGUIERE. 



St/non.Lingula, BBTTO. Encyc. Meth. I, 1792, tab. 250. CUVIEK, Tab. Elem. 1798, . . . ; Ann. Mus. I, 1802, 69. 



LAMK. Prodr. 1799, 89 ; Syst. Ann. 1801, 140. 



Pharetra, BOLTKN, Mus. Bolt. 1798, 2d ed. 1819, 111 (not Hubn. 1816). 

 Etym. Linyula, a little tongue. 

 Type. Lingula anatina, LAMK. 



Shell oblong or more or less oval, depressed, thin, gaping at each end, and 

 rounded or subtruncate in front, and more or less pointed at the beaks con- 

 sisting of alternate corneous and testaceous laminae, the former of which are fibrous 

 and the latter tubular ; composition largely phosphatic. Valves both moderately 

 convex, held together by the action of muscles; beak of ventral valve more 

 pointed and prominent than that of the other. Surface smooth, or marked by 

 concentric lines, sometimes crossed by radiating striae. Peduncle long, thick, 

 cylindrical, fleshy, and flexible. 



On the inner side of the shell of the typical forms of this genus, the marks of 

 the visceral sac and the scars of the complex muscular system occupy most of the 

 posterior half of the valves. In the dorsal or shorter valve, this visceral area has 

 a somewhat rhombic or suboval form, and in the ventral valve its outline is ovate- 

 cordate, or more or less flabelliform. The area thus designated is usually thicker 

 in both valves than other parts of the shell, especially in old examples, so as to 

 leave a slight impression on internal casts. 



Of the muscular impressions, the form and position of which have been noted, 

 there are twelve in the dorsal, and thirteen in the ventral valve. The scar of the 

 peduncular muscle is situated immediately within the beak of the ventral valve ; 

 and just in front of it is the scar left by the divaricator muscles (of Hancock = 

 posterior adductors of Woodward). At the anterior extremity of the visceral area, 

 in the middle of the same valve, are the four very unequal scars of the posterior 

 occlusor, and external and central adjuster muscles (of Hancock), which are so 

 arranged and impressed as to impart a more or less trilobate outline to the anterior 

 margin of the slightly more convex visceral area. Behind these, and just within 

 each lateral lobe of the visceral area, are situated, one on each side, the widely 

 separated anterior occlusor scars ; and still further back, we see on each side those 

 of the posterior adjusters, of which there are two on one side, and one larger on 

 the other. 



