332 [December, 1843. 



wider than the middle lobe. From Illaenus it may be distinguished by its 

 ovate form, want of reticulated eyes, the width of the lateral lobes, and the 

 profound lobes of the tail. 



The genus Thakops characterises the Trenton limestone formation of the 

 Lower Silurian group. 



Thaleops ovata. Obtusely ovate, surface minutely punctate; head very wide, 

 lunate, involuted ; eyes very prominent, rounded, smooth, placed on a line with 

 the angle in the middle of the side lobes; ribs flattened, smooth, without a bor- 

 der at the extremities where they are rounded and not expanded; post-abdomen 

 with the middle lobe convex, rounded and well defined at the extremity ; inferior 

 margin obtusely rounded. Length two-thirds of an inch: width of buckler 

 three-quarters of an inch. 



Locality. Mineral Point, Wisconsin, (blue limestone quarry,) Mr. Taylor. 



This rare trilobite may be studied as well in Mr. Brano's excellent model, as 

 in the natural specimen. It is probably a rare fossil that few naturalists will be 

 able to procure, and for this reason we would recommend the model to those who 

 are desirous to have a knowledge of the genus. 



Wherever many entire trilobites are found, a very quiescent state of the 

 waters wherein the rock which contained them was deposited, may be inferred 

 from the abundance of scattered bucklers and post-abdomens, which occur in other 

 strata where there is no evidence of violence on any of the delicate shells accom- 

 panying them ; the articulations of the abdomen were therefore held together by 

 a frail ligament, whilst the post-abdomen although ribbed in many species, was 

 in all composed of an inarticulated crust and thus is almost always preserved en- 

 tire in the rock. 



CRUSTACEA. 



Cytherina. 



Cytherin a fabulites. Ovate, reniform, smooth, with a slightly concave or 

 truncated cardinal margin, the extremities of which are angulated; end margins 

 rounded. Length, half an inch. 



Locality. Mineral Point, Wisconsin, (Trenton limestone.) 



SHELLS. 



STROPHOMENA. 



1. Strophomena deflecta. Semi-oval ; superior valve slightly concave, 

 deflected at the angles, the other valve reflected ; radii very closely arranged, 

 prominent, subequal, minutely crenulated ; inferior valve slightly depressed in 

 the middle ; cardinal area wide ; superior margin of the concave valve rather 

 elevated. Breadth, half an inch. 



Locality. Ibid, (Trenton limestone.) 



2. Strophomena recta. Semi-oval, with angulated slightly prominent 

 hinge extremities; radii strongly defined, numerous, minutely crenulated; su- 

 perior valve flat, with a prominent umbo ; inferior valve depressed, with a wide 



