BRACHIOPODA. 133 



FAMILY LINGULIDiE. 



Animal fixed by a pedicel passing out between the beaks of tiie valves. 

 Valves sub-equal, horny. 



No. 407. Lingula cuneata, Conrad. 



Slab, showing a number of specimens. Shell oblong; compressed; 

 dorsal valve somewhat shorter, horny, greenish. Tlie genus began in the 

 Cambrian and has continued to the present. 140 species fossil, and 16 living 

 Medina Sandstone, Medina, N. Y. 



No. 408. Lingula (Lingulepis) prima, Con, 

 Potsdam Sandstone, Keeseville, N. Y. 



CLASS TUNICATA. 



These " cloaked " mollusks, represented by the Ascidians and 

 Salpians, are invested in a leathery test which takes the place of 

 the shells of the other classes. This test is remarkable for con- 

 taining a considerable proportion of a substance identical with 

 cellulose. The blood is carried back and forth along the same 

 simple tube, as was supposed to be the case in the human system 

 before the time of Harvey. The nervous system consists of a 

 single ganglion placed near the oral aperture. 



There are various points of resemblance between the embryos 

 of Tunicates and those of vertebrates ; and some embryologists 

 are led to regard the Tunicates as degenerate vertebrates. 



They exist in all seas, from low water to a considerable depth. 

 On account of the absence of any consistent skeleton, they have 

 not certainly been found fossil. 



