CHAP. X. HYRIN^ AND UNIONINiE. Og^ 



species, four of which are from India_, and two from 

 Australia. They are short, roundly oval shells, with 

 the umbones well defined, and placed tow^ards the 

 middle: from this results their peculiar character. The 

 lateral teeth are short, and not a great deal longer than 

 the cardinal teeth ; these latter being strong and thick, 

 without being short : they are much crenated, yet 

 without any ridge ; so that they have the appearance of 

 being broken off. Their immediate affinity to Tridea 

 is proved by the Na'ia elongata, from New Holland, 

 having the form of one type, and the teeth of the other. 

 This interesting species, once the property of admiral 

 Bligh, was purchased by us at his sale. The pecu- 

 liarities of Lamarck's Unio Australis (^Hyridella Sw.), 

 and its affinity to Iridea, lead us to arrange it as the 

 anodontine type of the Hyrian^, although it may pos- 

 sibly be an aberiant example of Iridea : the bosses, how- 

 ever, are not striated, and the wdiole shell has very much 

 the aspect of an Anodon. Of the fifth type, connecting 

 this sub-family to the next, we know nothing more than 

 what has been already stated. As the remaining genera 

 of the Unionidce are neither rich in the number of their 

 forms, nor abundant in their species, we shall mention 

 them wath more conciseness. 



(266.) According to the views here taken of the 

 Hyrin^, we may thus compare the four genera, just 

 defined, with the circle of the Unionin^. 



Analogies of the Hyrin.e and the Unionin^. 



Genera of the j„„7^„,-„o Genera of the 



HyriiiLe. *' Lnioni/ue. 



r^.om.T.. CShell trigonal: posterior side^ zc^t.. 



Castalia. J truncate and short. j ^^«"^- 



Hyria. Posterior side lengthened. Unio. 



. ? POTOMIDA, 



TT ^ ("Compressed, broad: bosses not7 t, .......... 



Hyridella. I striated, retuse ; teeth slender.] Lymnadia. 



C Lengthened oblong ; bosses small; "i 



Iridea. < inner cardinal teeth very small, > Mysci, 

 C. almost obsolete. J 



