SALTER PELTOCARIS. 91 



In Peltocaris this habit was certainly absent, and the whole contour 

 indicates a flat, shield-shaped carapace, though divided into three 

 parts. In Dithyrocaris (fig. 8) the sutures themselves are lost, as 

 they are in Nebalia (fig. 12). 



Peltocaris would seem to be, in some sort, intermediate between 

 the shield-bearing Dithyrocaris, whose valves are soldered into one, 

 and the bivalved genera. And, if we had the body preserved, we 

 might probably find a still nearer relation to a more ancient genus, 

 found in the Lingula-flags, I mean the Hymenocaris (fig. 3), now 

 becoming pretty well known. 



I beg leave to express what appears to me to be the series of 

 affinities of this and other known Phyllopodous forms in a diagram, 

 which shall at the same time give their chronological order. The 

 numbers correspond with those on the woodcut. 



Pig. 3. Beginning with what I must call the more generalized 

 type, Hymenocaris. In this the shield is neither flat nor bivalved, 

 but simply bent ; and without any rostrum. There is a medium 

 number of free body-segments ; while the appendages of the tail are 

 irrelatively multiplied. (Lingula-flags or Primordial Zone.) 



Pig. 4. A form (Peltocaris) in which the carapace is marked out, 

 as it were, into three portions, not yet intended to enclose the 

 animal. The body and appendages are not known ; but a Crustacean 

 certainly existed in the Llandeilo-flags in some respects intermediate 

 between Hymenocaris and Ceratiocaris, and we may perhaps be justi- 

 fied in provisionally referring to Peltocaris such tracks as those de- 

 scribed below (page 93). 



Pig. 5. Peltocaris (?)*. — Known only by tracks made by a double 

 instrument which may have been part of a more complex tail (see 

 p. 93). 



Pig. 6. Ceratiocaris. — Here the rostrum is narrowed, but still not 

 soldered to the carapace. And the valves have free motion, and 

 partly enclose the body. The tail-appendages are three ; the telson 

 being enlarged to make the third and larger prong. 



Pig. 7. A very imperfectly known form of immense size occurs in 

 the Lower Devonian and Upper Silurian ; but all that is known of it 

 is that the carapace is undivided and reticulate (Dictyocaris). 



Pigs. 8 to 12. Prom this point the larger Phyllopods (for, though 

 I have inserted them, figs, a to i, it may be a question yet whether 

 Leperclitia and its allies be undoubted Phyllopods) diverge (in a 

 geological sense only) into two groups ; the one becoming more 

 decidedly shield-shaped, and with exserted abdomen and large caudal 

 styles ; the other starting at once with a bivalved carapace, com- 

 pletely enclosing the small abdominal segments and their minute 

 appendages (Esiheria). 



I do not pretend to have shown every link, but have given all I 



* I hope I may be allowed to insert in this figure (which is only a diagram) 

 the caudal appendages at least of the hypothetical Crustacean whose existence in 

 the Llandeilo strata I cannot doubt (see next paper). It might be too much 

 to surmise that it belonged definitely to Peltocaris, or that the P. Harknessi, a 

 Llandeilo species, actually produced the tracks. 



