11 KI'TI I.IAN AGE JURASSIC PERIOD. 91 



arranged. The following arc tin- names of the genera most usually admitted by 

 late writers. \i/.. : /'///.. Ainxlutitn. lii/xmnitHloiitn, Miii-i/Ki-itumi, Mmtm-ninlylltnt, and 

 llmli'iln. I'm!'. Ai^ai/.. ho\\e\er. who lias disserted, with much care, many of our 

 Aim-rii -an >pi-cies. timls that they present marked differences in the arrangement 

 of the .yills, and the position of the e<j, r s in the same, as well as in other anatomical 

 details, from winch lie is led to the conclusion that there are at least twenty-two 

 distinct genera amongst our species usually referred to Unlo, Atiodonta, Alasmo- 

 i/onfd, and Mnr</m-!/<nt<t. For these groups he has adopted the following names, 

 vi/.. : Dymomia, Ag. ; Scalenaria (Raf.), Ag. ; Truncilla (Raf.), Ag. ; Lamprilu, 

 Rat'.; Ciiiitln/rin. Swainson ; l-'.u ri/uin, Raf. ; Mdaptera, Raf. ; Alasmodonla, Say; 

 Obornriii. Kaf. ; Mi< nuiii/ii, Ag. ; C'.'//"'".'/' "'" Ag. ; Plagicla, Raf.; Orthonymus, 

 Ag. ; Tri't'M/nitiit, Ag. ; Qumlniln, Raf. ; Rotundaria, Raf. ; Complanaria, Swainson ; 

 Pleurobemn, Kaf.; I'n iopsis, Swainson ; Margaritana, Schumacher ; Hemilasterna, 

 Raf., and L'nin. Ketz. 1 



Mr. T. A. Conrad, of Philadelphia, who admits many of these groups as 

 suli^enera under l'ni<>, also proposes to adopt the following additional subgenera, 

 mainly tor the reception of foreign species, viz. : NoduJaria, Con. ; Iridea, Swainson; 

 Mi/tt'-a, Tnrton ; I.<i/i<;<,l<tr!<i,Con.; Coflatura, Con. ; Cunicula (Sw.), Con. ; Glebula, 

 Con.; UnioiiK i -us, Con., and Theliderma, Swainson. The following he proposes as 

 full genera: Citcumaria, Con. ; I/yriopsis, Con., and Monodontina, Con. 



As an example of the widely different views entertained by authors in regard to > 

 the classification of these mollusks, we should remark that Dr. Isaac Lea, who has 

 jjivcn more attention to the study of the Unionidce than perhaps any other person, 

 includes the whole, along with some others not generally admitted in this family, 

 under two generic heads, for which he adopts the names Margaron and Platiri*. 

 Under the first of these he ranges as subgenera, Triqwtra, Prisodon, Uhio, Margari- 

 tana, ManocondylcEa, Anodonta, and Dipsas, ; and under the second Iridina, SpatJm, 

 and Mycetopus? 



Mr. Niclin went still farther in this direction, and included Unio, Anodonta, Alas- 

 modonla, Iridina, Dipsas, Syria, and Costal ia as members of a single genus ! s 



The family Unionidte has a wide geographical distribution, but is most numer- 

 ously represented in the streams of North America. Although apparently repre- 

 sented as far back as the Jurassic period, it is pre-eminently characteristic of the 

 present epoch, since the species and genera are far more numerous now than they 

 were during any of the past geological periods. The existing species also present 

 much greater diversities of form and ornamentation, and sometimes attain larger 

 sizes than are known to occur amongst those now extinct. 



1 Wcigmann's Arch. 1852, p. 41. * Synopsis Naldes, 3d ed. Phila. 1852. 



Trans. Phil. Soc. VIII, p. 398. 



