MEGALOPIDEA. 487 



the projecting front ; and if Leach's species had lamellar posterior 

 tarsi, the species of Cyllene would be true Megalopae. There is, also, 

 another point of resemblance, that is, a tendency to being spinigerous 

 in other parts of the body : the M. armata has a spine to the posterior 

 margin of the carapax, while one of our species has two long spines to 

 the posterior part of the thorax, under side. None of the other 

 genera partake of this peculiarity. 



The adopted genera of Megalopidea, have the following charac- 

 ters : 



G. 1. MARESTIA, Dana. Carapax fronte tricuspidatus, sed rostrum valde deflexum 

 et frons superne visus medio non acutus. Pedes 8 postici ad basin infra non ar- 

 mati; 5ti super carapacem saepe restantes, depressione ad eos recipiendos parce 

 concava; tarso styliformi, compresso, spinis infra armato, paris postici setis longis 

 apice instructo. 



G. 2. MONOLEPIS, Say. Carapax fronte tricuspidatus, sed rostrum valde deflexum 

 et frons superne visus medio non acutus. Pedes 8 postici ad basin infra non 

 armati ; 5ti minores, super carapacem ssepe restantes, depressione ad eos recipi- 

 endos abrupta, tarso styliformi inermi, depresso, ,postico setis longis apice in- 

 structo. Sterni fossa abdominalis marginibus subacutus et prominens. 



G. 3. MEGALOPA, Leach. Carapax fronte rostratus, rostro vix deflexo, acuto. 

 Pedes 8 postici ad basin infra uni-spinigeri ; 5ti minores, tarso styliformi. 



G. 4. CYLLENE, Dana. Carapax fronte simpliciter rostratus, rostro vix deflexo. 

 Pedes 8 postici ad basin infra uni-spinigeri; 5ti 4tis parce minores, tarso lamel- 

 lato, versus apicem elongate setigero. 



G. 5. TRIBOLA, Dana. Carapax fronte horizontals, tricuspidatus, rostro (vel cuspide 

 mediana) cuspidibus externis vix longiore et supra sulcato. Antenna internae 

 inter rostrum et cuspides externas aperte inflexae. Pedes duo postici minores, 

 tarso styliformi, setis longis non instructo. 



GENUS MARESTIA, Dana. 



THE genus Marestia includes Monolepis spinitarsus of Say. The 

 distinction, in the aspect of the species, between this genus and Mono- 

 lepis is very striking. The carapax is nearly flat above, and much 

 less thick or obese. The sternal fossa is deep, with slender trenchant 

 borders, in Monolepis, while in Marestia, it is much less deep, and the 

 borders are rounded, or more gradually flaring. The depression on 

 the surface of the carapax for the posterior legs is another point of 

 distinction ; moreover, the tarsi are depressed and unarmed in Mono- 



