Bkede.] Carboniferous Invertebrates. Ill 



PLACUNOPSIS. 



Morris and Lycett, Mon. Foss. Great Oolite, ( 1853). 



Placunopsis carbonaria. Plate XX, fig. 18. 



P!acuno})s>$ carbonaria Meek and Worthen, Proc. Chic. Acad. Sci., i, p. 

 13, (1866); Geol. Surv. 111., v, p. 57S, pi. xxvn, f. 2, (1873). 



Meek and AVorthen's description: "Shell orbicular, com- 

 pressed subhemispherical, extremely thin. Upper or right 

 valve irregularly convex, sometimes rather gibbous ; hinge 

 margin straight, generally equaling about one-third to one- 

 half the diameter of the valves, and usually showing slight 

 disposition to develop small, obtuse ears at the extremities ; 

 beak very small, sometimes compressed and nearly obsolete, 

 marginal, but not projecting distinctly beyond the cardinal 

 border, located at the middle of the hinge, and showing a slight 

 forward curvature. Under valve flat or conforming to the in- 

 equalities of the surface to which it was attached ; beak mar- 

 ginal, and very nearly obsolete. Surface of both valves marked 

 with irregular, undulating or interrupted radiating lines, with 

 broad, faint, irregular concentric wrinkles, and a set of obscure 

 striae of growth. Often there is also an entirely independent 

 series of parallel ridges, crossing the umbonal region, or some- 

 times the whole surface obliquely and partly, or sometimes 

 almost entirely, obliterating the other markings. Diameter of 

 the largest specimen found, from the hinge to the pallial mar- 

 gin, 1.30 inches; oblique parallel ridges uniformly numbering 

 seven to eight in 0.2 inch." 



Range and distribution : Upper Coal Measures ; Topeka. 



by (/<t labial palpi; /, foot: i, anus; m, cut edge of the mantle (after Nicholson). 2. Left valve 

 of Cytheria chione i after Woodward) : A, anterior margin ; />, posterior margin; C, ventral 

 margin or base : u, umbo; A, ligament: <•, cardinal tooth; 'f, lateral teeth: a, anterior adduc- 

 tor; "'. posterior adductor : /;, pallial line : 8, pallial sinus caused by the retractor muscles of 

 the siphons (after Nicholson i. 3. Diagrammatic representation of the anatomy of a siphonate 

 pelec\ pod : the left valve and the left mantle Lobe are removed, and the siphons are cut short: 

 >/, umbo : co, cartilage pit : o, the mouth ; l/>, labial palpi : ", stomach, surrounded by liver : i, 

 intestine perforating the heart: /•, rectum, terminating in the anus; ad, anterior adductors, 

 pi. posterior adductors: //, supraesophageal or cerebral ganglion (the mouth is a little 

 displaced upwards, so that the ganglion comes to lie below the gullet instead of above it) ; n 1 , 

 parieto-splanchnic or branchial ganglion ;/, foot ; /r, cut edge of the right mantle lobe; rs, 

 retractor muscles of the siphon; hr, branchin- <>f the left side; g, renal organ; 8, inhalant si- 

 phon \ t 1 , exhalant siphon (after Nicholson). 



