14(5 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Family BELINURID^ Packard. 

 Pkestwiohia dan^ )Meek) PI. V,figs. 3, S''; VI, 1, 1", 2, 2\ 



}ittnini(ni>i dance, Meek and Wortheu, Proc Acad. Nat. Sc, Phil., llarch 1865, Rt. Geol. Surv., 111. ii, 395, 



1860. 

 Prestwichia datuv Meek, Amer. Jouru. Sc, 2d ser., xUii, 257, 1867. 

 Enproops danw, Meek, Amer. Journ. Sc, xliii, 394, 1867. 



Meek and Worthen, Rt. Geol. Surv. 111., iii, 547, 1868. 



Packard, Amer. Naturalist, March, 1885. 



Head aud abdomen (urosome), iii the largest specimens, of the same length; in younger speci- 

 mens the head is rather shorter than the abdomen ; head about one-third as long as broad ; geual 

 spine about two-thirds as long as the head, and turning at nearly a right angle with the straight 

 hinder edge of the cephalic shield ; the spine as a whole is directed somewhat outward, nearly 

 reaching a point about opposite the hinder edge of the third abdominal segment. Median lobe 

 of the head or glabella, rather deeplj' excavated in front; at the bottom of the excavation are 

 situated traces of the simple ej^es, which have the same situation and shape as in Limulus. The 

 small compound eyes are situated on the outer anterior angle made by the sides of the glabella 

 and are of nearly the same relative size and in the same general situation -as in the larval Limulus, 

 though placed a little nearer the front margin. The eyes themselves are small, oval and promi- 

 nent. The sides of the glabella are produced behind into a sharp spine, projecting backwards over 

 the base of the abdomen. 



The abdomen (or urosome) is from one-fourth to one-third broader than long, and is composed 

 of eight distinct segments, including the caudal spine; the body of the abdomen is full, convex, 

 and distinctly trilobate, the median or cardiac lobe being in general about a third narrower than 

 the lateral lobes or pleura, and contracting in width towards the fifth segment. The sutures be- 

 tween the segments on the lateral lobes are very distinct, being raised, narrow ridges, prolonged 

 into and forming the hinder edge of the long, sharp, slightly curved, lateral spines; of these lateral 

 spines those on the first and second segments are the narrowest and most acute, that on the seventh 

 the widest and most obtuse. In the cardiac lobe the third abdominal segment bears a high rounded 

 tubercle, and there is one about twice as large on the sixth segment ; those on the other segments 

 are small, and in most of the specimens there are traces only of those on the third aud sixth seg- 

 ments. The caudal spine (representing the eighth abdominal segment) is somewhat enlarged at 

 the base; it is three-cornered in section, much as in Limulus, the surface is smooth, and it is about 

 three-fourths as long as the abdomen. 



Length of eutire body (largest specimens), 60""" ; breadth, 53"'™. 



Length of cephalic shield, 24™"' ; breadth, 53™™. 



Length of lateral cephalic spine, 15™™ ; breadth, near base, 3.5™'°. 



Length of abdomen (urosome) (not including the caudal spine), 23'"™ ; breadth 35'"'". 



Length of longest lateral abdominal spine, 6™'". 



Length of caudal spine (telsou), 15'"'". 



The smallest specimen is 10'"™ in length, and 12""" in width, the caudal spine being less than 

 one half as long as the abdomen. 



Description of the cephalic appendages. 



In a nodule from Mazon Creek received from Mr. J. C. Carr, containing the remains of a speci- 

 men 55™™ across the shield (PL VI, figs. 2, 2"), the cephalic appendages are more or less distinctly 

 preserved. Of the first pair there are faint traces, the two small limbs Ijing parallel to each 

 other aud in the same position as in the larval Limulus, and of nearly the same proportions. The 

 impressions of the succeeding limbs are distinct ; the second third, fourth, and fifth pairs are of 

 the same size, the fifth pair being perhaps a little longer, as the tips extend near the edge of the 

 cephalic shield. All four pairs, i. e., second to fifth, are chelate, the forceps being well developed 

 and plainly visible in the third and fourth pail's, as these limbs are turned on their side; the fifth 

 pair are undoubtedly chelate, but lie so that the outline is a simple point. The sixth pair differs 



