EXPLANATION OF PLATES XXVIII AND XXlX—coKiimeed. 



lamella; at?^., parabraucliial stigmata on the sternal surface; d., bases of the 

 lainellse supposed to be seen through the axis of the appendage by trans- 

 parency. 



Fig. 2 a. — Hypothetical form leading on to Limulus. Letters as before, 

 excppiing R' R" pointing to tiie distal region of the appendage now pro- 

 longed beyond the lamelligerous region, and ex. the exite. The para- 

 branchial stigmata are now in proportion to the size of the appendages 

 much smaller than in fig. 1. The median sternal tubercle seen in fig. 1 has 

 now grown up into a fold uniting the bases of the appendages. 



Fig. 3 a, — The form realised in Limulus. The appendages no longer 

 diverge, but converge, and the median sternal fold or lobe widely unites 

 their proximal segments, and overlaps the parabranchial stigmata, s(^., 

 which are indicated by dotted lines, as though showing through the sternal 

 fold by transparency. 



Fig. 2 b. — Hypothetical intermediate form leading from that represented 

 in fig. 1 to the condition realised in Scorpio. The letters have the same 

 significations as in fig. 1. The parabranchial stigmata are now greatly 

 enlarged, and the appendages reduced in size, so that the latter hang, as it 

 were, from the anterior margins of the former. 



Fig. 3 b. — The condition found in Scori)io and the pulmonate Arachnids 

 is shown. The margins of the parabranchial stigmata have contracted, 

 enclosing within the sunken sternal surface the reduced lamelligerous 

 appendages. On the right hand side of the figure the appendage is repre- 

 sented as though the integument covering it in were quite transparent. It 

 has rotated on its base line, so as to present what was the concealed or 

 posterior face. 



Figs. 4, 5, 6. — ^Diagrams of sections of the sternal wall of the three 

 stages drawn in figs. 1, 2 b, and 3 b. ; ch., the integument ; E., axis of the 

 appendage ; /., lamellae of the appendage ; sit/., cupped surface of the 

 sternum or parabranchial stigma ; w., thoraco-brauchial muscle attached to 

 the cupped integument. 



