104 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 1 37 



segment of the leg, drawing back into the lateral wall, shortened to 

 such an extent that it almost disappeared, except on the side where it 

 has formed, up to a certain level, the mesal wall of the gill chamber 

 of the decapod. 



Our figure 2 shows the internal side of this wall above the third 

 right pereiopod of Penaeus}^ From the direction of the lines 

 which margin or run across the precoxopodian wall, we have the im- 

 pression that it penetrated into the lateral wall like a wedge, pressing 

 back the primitive wall more toward the middle of the segment than 

 at the ends. Therefore, at these two extremities, the old wall could 

 remain rather close to the coxa.^^ The coxa itself has grown like 

 a wedge toward the lateral wall at its anterior angle (a) which is ele- 

 vated compared with its posterior angle (/3) ; a third angle (y) exists 

 on its proximal side ; the upper frame of the coxa is thus triangular. 

 The proximal side of the coxa is dihedral in keeping with a certain 

 overlapping of the leg bases ; it has an oblique anterior side against 

 which the back of the coxa of the preceding leg can be moved and a 

 posterior side which runs along the margin of the sternite.^® 



At each angle a and ^S of the coxa there is an articulation with the 

 pleuron; it is a kind of "suspension" of the coxa which really re- 

 sembles that of the last two pairs of coxae of the Machilidae (Car- 

 pentier, 1946, fig. 6). It is one more reason why we consider the 

 Machilidae as having preserved certain resemblances with the 

 Crustacea. 



Above the articulation of the angle a an apodeme {ap), which we 

 have every reason to homologize with the pleural apodeme of the 

 insects, arises and bends backward in Pcnaeus as well as in the last 

 two thoracic segments of the Machilidae. The apodeme does not pre- 

 sent any process in Penaens, but we find a rudimentary one in Amalo- 

 penaetis}'' The apodeme divides the pleuron into two regions, 

 anterior and posterior, which include the equivalents of the episternum 

 and of the epimeron of the Pterygota. However, these regions are 



1* We have chosen this third leg as typical ; it is far from the head and it is 

 the last one of those which, even at their base, are not influenced by the spe- 

 cialization of the genital region. 



15 This is only a general impression. We shall not be able to go into the 

 details of the specialization which has affected the leg base, especially on the 

 proximal side. Here we only suggest a few guiding marks. 



1^ Compare with the tranversal sections of the coxae of a Camhariis on 

 fig. 43 A of Snodgrass (1952). 



I'' Amalopenacus valens S. I. Smith (we used some of the well-preserved 

 specimens which had been brought back years ago by Prof. D. Damas from 

 his expedition with the Armaucr Hansen, 1922). 



