Genus Clastotoechus, new 175 



Clastotoechus diffractus (Haig) 176 



Genus Allopeirolisthes,nevf 178 



Allopetrolisthes angulosus {Gninn) 180 



Allopetrolisthes punctatus (Guerin) 182 



Allopetrolisihes spinifrons {Yi.MWnt'EA-wavAs) 185 



Genus Euceramus Stimpson 187 



Euceramus panatelus Glassell 188 



Euceramus transversilineatus (Lockington) 190 



Genus Minyocerus Stimpson 193 



Minyocerus kirki Glassell 193 



Genus Porcellana Lamarck 196 



Porcellana hancock'i Glassell 198 



Porcellana cancrisocialis Glassell 200 



Porcellana paguriconviva Glassell 203 



Porcellana corbicola, new species 205 



Genus Pisidia Leach 207 



Pisidiamagdalenensis (Glassell) 209 



Genus Megalobrachium Stimpson 212 



Megalobrachium poeyi (Guerin) 214 



Megalobrachium smithi (Glassell) 215 



M egalobrachium s'lnuimanus (Lockington) 217 



Megalobrachium gart/ii Haig 220 



Megalobrachium erosum (Glassell) 222 



Megalobrachium perwvianum, new species 224 



Megalobrachium festai (Nobili) 226 



Megalobrachium tuberculipes (Lockington) 227 



Genus Ulloaia Glassell 229 



Ulloaia perpusillia Glassell 230 



Genus Polyonyx Stimpson 232 



Polyonyx confinis, new species 234 



Polyonyx quadriungulatus Glassell 236 



Polyonyx nitidus Lockington 239 



Literature Cited 242 



Appendix I : Tables of Material Examined 253 



Appendix II: A Summary of the Commensal Relationship of Porcellanids 



and Their Hosts in the Eastern Pacific 346 



Explanation of Plates 349 



vii 



