BRACHIOPODA 



171 



CLASS I. INARTICULATA 



The valves are not provided with teeth, but are held 

 together by the muscles and mantle. The intestine 

 is long and ends in an anus. There is no brachial 

 skeleton. 



ORDER I. ATREMATA 



The peduncle passes out between the umbones, the 

 opening being shared by both valves. E.g. Lingula. 



Obolella. Shell ovate or sub-circular, lenticular. Ventral 

 valve with a solid umbo, and a small area with a groove in the 

 middle ; one pair of long muscular impressions extend from near 

 the hinge-line to the middle of the valve, between these are a pair 

 of small impressions, and near the hinge-line a third pair of small 

 impressions. Dorsal valve with a minute umbo, a small area, an 

 internal median ridge, and two long muscular impressions diverging 

 widely. Cambrian. Ex. 0. crassa. 



Lingula (tig. 76). Shell thin, nearly equivalve, compressed, 

 elongate-ovate or quadrilateral, ta- 

 pering towards the umbones, slightly 

 gaping at the extremities. Dorsal 

 valve a little shorter than the ven- 

 tral. Hinge-line slightly thickened. 

 Twelve muscular impressions in 

 each valve, but usually indistinctly 

 marked. Surface of shell smooth, or 

 concentrically or radially striated. 

 Peduncle long, passing out between 

 the umbones. Shell composed of 

 alternating layers of calcareous 

 and chitinous material. Cambrian 

 to present day. Ex. L. anatina, 

 Recent ; L. ovalis, Kimeridge Clay. 



Lingulella. Distinguished 

 from Lingula by the presence, in the ventral valve, of a high hinge- 



Pig. 76. Lingula anatina, Re- 

 cent. Interior of valves show- 

 ing muscular impressions. A, 

 ventral valve. B, dorsal valve. 

 u, umbonal muscle; t, trans- 

 medians ; c, centrals ; a, an- 

 terior laterals ; in, middle 

 laterals ; e, external laterals. 

 a 2 . 



