INTRODUCTION. XXXi 
Paciric. ATLANTIC. 
1. Philypnus maculatus. P. dormitator. 
2. Eleotris picta. FL. pisonis. 
3. Dormitator latifrons. D. maculatus. 
4. Gobioides peruanus. G. broussonetii. 
5. Gerres simillimus. G. cinereus. 
6. Gerres californiensis. G. harengulus. 
7. Gerres peruvianus. G. olithostomus. 
8. Gerres lineatus. G. plumieri. 
9. Centropomus medius. C. pectinatus. 
10. Centropomus viridis. C. undecimalis. 
11. Centropomus robakto. C. ensiferus. 
12. Centropomus unionensis. C. altus. 
13. Pomadasis bayanus. P. crocro. 
14. Mugil thoburni. M. ineilis. 
15. Alurichthys scutatus. LK. tsthmensis. 
16. Ailurichthys pinnimaculatus. Ad. bagre. 
17. Galeichthys guatemalensis. G. assimilis. 
18. Arius liropus. al. melanopus. 
In several cases there is identity in the number of fin-rays, scales (when present), 
and gill-rakers between the members of a pair of species. This applies to the 
following :—Dormitator maculatus and D. latifrons, Gerres lineatus and G. plumert, 
Centropomus robalito and C. ensiferus, Centropomus unionensis and C. altus, Mugil 
thoburnt and M. incilis, Hlurichthys scutatus and 4. isthmensis, Galeichthys guate- 
malensis and G. assimilis, Arius melanopus and A. liropus. 
In other cases there is agreement in the number of fin-rays and of gill-rakers, but 
not of the scales (e.g., Gerres simillimus and G. cinereus, Gerres peruvianus and 
G. olithostomus, Centropomus medius and C. pectinatus, Pomadasis bayanus and 
P. crocro). In the last pair the slight difference in the size of the scales is uniform ; 
a longitudinal or transverse count on any part of the body will probably give a larger 
number for the Pacific form. In Centropomus medius and C. pectinatus the scales 
differ in number only above the lateral line, especially anteriorly, where they are 
always smaller in the Atlantic species. Gerres peruvianus is identical in scaling with 
G. olithostomus, except in the thoracic region, where it has distinctly fewer and larger 
scales, whilst the only tangible difference in the scaling of Gerres simillimus and 
G. cinereus is the constant presence in the latter of 5 or 54 instead of 4 series of scales 
above the lateral line. 
Gerres californiensis and G. harengulus are two species which agree in the number 
