Contributions to Palaeontology. 



173 



Whenever this locality, and the region about it, shall be more 

 fully investigated, we may confidently predict that additions of 

 much value and interest will be made to the primordial fauna of 

 he Upper Mississippi valley. 



GEXUS AGLASPIS ( n. g.). 



The investigations in the upper part of the Lower sand- 

 stone of the Mississippi valley have furnished me with the 

 carapace, some fragments of the thoracic articulations, aud 

 what appears to be a caudal spine of a new and remarkable 

 crustacean, for which I have proposed the name Aglaspis. 1 



Generic description. Carapace wide, sublimate, or approach- 

 ing semicircular ; its superior crust not separable into 

 parts by suture lines ; a sinus in the middle of the front, 

 and preserving some evidence of trilobation in the pos- 

 terior part. Eyes anterior to the middle, large and 

 prominent. 



Thorax probably subtrilobate, composed of several articu- 

 lations, which are recurved at their extremities. Posterior 

 or caudal portion more elevated in the middle and 

 strongly arching : the caudal extremity probably furnish- 

 ed with an elongate spine. Texture punctate 

 The only species known, in its carapace reminds one of Limulus; 

 and though the resemblance is not so apparent in the character of 

 the eyes, yet we find that these organs occupy the relative position 

 of the two oculiform spots on the anterior part of the carapace of 

 that animal. The segments of the abdomen, which in that genus 

 are anchylosed, are here free, in their anterior members at least; 

 while the posterior ones are highly arched and closely united. The 

 associated spines, of the same texture as the other parts, can scarcely 

 have had any other relation to the body than the caudle spine of 

 LlMULlTB; and the Aglaspis, with its broad depressed-convex 

 carapace and its anterior eyes, was furnished with a long caudal 

 spine as in the modern genus. 



x See Canadian Naturalist and Geologist, Vol. vii, p. 443, December 1862. 



