55 



ASAPHUS PLEUROPTYX.* Cast No. IS. 



Corpore depresso; cute coriacea tuberculis mini- 

 mis; costis striatis; cauda acuta, brevi. 



This species like most other specimens of this 

 genus, in our cabinets, is decapitated every other 

 part, however, appears to be in a good state of pre- 

 servation. 



The articulations of the abdomen and tail, which 

 cannot readily be distinguished from each other, are 

 seventeen in number. The middle lobe is flat, and 

 regularly tapers to an obtuse lip; it is marked on 

 each side with longitudinal impressed lines or little 

 grooves. The costal arches on their upper side have 

 a deep and narrow channel, running through their 

 whole course. The costal arches of the abdomen 

 have no membranaceous expansion beyond their ter- 

 minations; this organization is only visible immedi- 

 ately below the end of the middle lobe, where it quick- 

 ly finishes in an acute point. A large portion of the 

 crustaceous shell remains, and is covered with dis- 

 tinct granulations; those on the tail are the least 

 obvious. 



This species approaches very near the A.caudatus, 

 but the grooves on the middle lobe, the smallness of 

 the costal arches, and the limited extent of the mem- 

 branaceous expansion round the lower portions of 

 the shell, will sufficiently distinguish it. 



* From the Greek word for " grooved ribs." 



