2 6o THE APODID^E PART n 



(3) We have further the fact, already mentioned, 

 that the Ostracoda are found among the Trilobites in 

 the Silurian strata, and may thus well have been a 

 modified Trilobite form. 



It is, however, clear that these arguments do equally 

 well to establish a deduction of the Ostracoda from a 

 primitive Phyllopod with a developing dorsal shield. 

 We have only to assume that both the head- and 

 dorsal shields were bent along the dorsal middle line. 

 The extraordinary likeness between the shells of some 

 of the early Ostracoda (e.g. Leperditia) to the shells 

 of such Phyllopods as Ceratiocaris Salteriana make a 

 Phyllopodan origin for at least some of the Ostracoda 

 very probable. We have further satisfied ourselves 

 by dissections, that at least in some Ostracoda the 

 ligament uniting the two halves of the shell runs back- 

 wards posteriorly beyond the point of junction of the 

 abdomen with the shell. We do not, however, give 

 up our first impression that some of the Ostracoda are 

 deducible from Trilobites. In addition to such a 

 significant form as that given in Fig. 57/>, we would 

 call attention to the fact that the shells of many early 

 Ostracoda are marked by lobes and grooves which 

 Barrande compared to the glabella and intervening 

 furrows, &c., of Trilobites. The presence also of the 

 " ocular ' tubercle on each shell in some Ostracoda 

 may well signify what its name implies ; the ocular 

 tubercle of the original Trilobite showing just as well 

 on the folded, as on the flat, head-shield. The part 

 played by the habit of rolling up will again be referred 

 to. 



