ZOOLOGY. C09 



or only with feeble indications of tbem in tbo bingo; two equal muscu- 

 lar impressions; pallial line entire. Palaeozoic. 



" Oril. '2. Desimodonata. Teetb of tbe binge none or irregular, con- 

 nected witb tbe ligamental processes; two equal mnscular impressions; 

 pidlial line sinuated. Pholadomyidcc^ CorbuUdcv, Mjjifla\ Auatinidce, 

 Mactridcc, Papkiidiv, Glycymcridcv, {f) ISolenidce, and all Tubicola. 



" 'Jrd. 3. Taxodonta. Teetb of tbe binge numerous, not differen- 

 tiated, in a straigbt, arcuated or angular row ; two equal muscular im- 

 joressions. Arcida\ Nuculida'. 



" Ord. 4. Heterodonta. Teetb of tbe binge few, distinctly separated 

 as cardinal and lateral, alternating, exactly tilling tbe i)its of the o^)- 

 posite valve ; two equal muscuhir impressions. UnionidcCj Cardiniidcc, 

 Astartidce, Crassatellidcv, Megalodontidw, Chamidw, Tridacnidce, Eryoi- 

 nidcc, Luciiiida\ Cardiida\ Cyrenidw, Cyprinidcc, Veneridcc, Gnathodon- 

 tidci', TeUinidcv, Bonacida'. Tbe Trigoniidw will form a distinct sub- 

 order. 



"Ord. 5. Anisomyaria, or Dysodonta. Teetb of tbe binge none or 

 irreguliir; two very inequal or only one muscular impression; pallial 

 line entire. 



(rt) Heteromyaria : AvicuUdce, Mytilidw, Prasvnidw, Pinnidcc. 



(b) Mouomyaria: Pectinidw, Spondylidcc, Anomdidw, Ostreidcu.^^ 



{Zoological Record^ 1SS3, pp. 8G and 87, JMoll.) 



Dr. von Martens, certainly a most competent judge, bas expressed 

 tbe opinion tbat tbis classification "bas several advantages in com- 

 parison witb liitborto accepted classifications: (1) tbe distinction of a 

 limited number of natural types, instead of an artificial separation into 

 Monomyaria and Dimyaria, or Asipbonida and Sipbonida ; (2) tbe union 

 of tbe Heteromyaria and tbe Monomyaria into one common cbief di- 

 visiou;" and (3) "tbe constitution of a special cbief division for tbe 

 Arcidte and Xuculidae, as tbese families offer very peculiar cbaracters 

 as well in tbe sbells as in tbe gills and foot." 



Tbe liresent recorder is unable to appreciate any superiority of tbe 

 new arrangement over tbat based on the muscles, and there are cei- 

 tainly more excei)tions of tbe contents of tbe so-called "orders" to tbeir 

 diagnoses than in tbe case of the groups defined by tbe muscles. As 

 Dr. Paul Fischer bas already remarked (Journ. Concb., v. 24, p. 121), a 

 single family (tbe Unionidn?) bas representatives not only deviating 

 from tbe diagnosis of the including "order" (Heterodonta), but exhibit- 

 ing cbaracters assigned to two others, Anodonta recalling tbe Paheo- 

 concba; or Cryptodonta, and Pleiodon tbe Taxodonta. Undoubtedly 

 Dr. Keumayr knew of tbese exceptions and regarded tbe forms an<l 

 questions as derivatives from tbe normal Heterodonta. Dnf the <lif 

 fcrences fi'om the type and assumption of cbaractcristicis of others 

 nevertheless exist. For this, if for no other reason, the value of sneh 

 characters for orders is nullified. There is also a want of co-ordinaiion 

 between the binge structure and modifi(rations of other parts which 

 S. Mis. 33 39 



