Dr. Wheelton Hind — Aviculopecten semicosiatiis,^Forilock, s^j. 59 



III. — Note on the Characters of the Hinge Plate in 



Aviculopecten semicostatus, Portlook, sp. 



By Wheelton Hind, M.D., B.S,, F.R.C.S., F.G.S. 



ME. JOHN SMITH, of Kilwinning, has been fortunate enough 

 to obtain from the Lower Limestone series of Auchenmade, 

 Ayrshire, a specimen of the left valve of Aviculopecten semicostatus, 

 Portlook, sp., which shows the hinge plate and interior most perfectly 

 preserved. Hitherto a study of the hinge plate of Aviculopecten has 

 been made from specimens which were merely casts of the interior,. 

 and consequently it has been impossible to observe that portion of the 

 hinge plate immediately interior to the umbo. The Auchenmade 

 specimen advances our knowledge of the anatomy of the genus, and 

 establishes a character denied for it by its author, M'Coy. 



We now know that Aviculopecten possessed a median cartilage pit, 

 in the centre of the elongate, somewhat hollow, flattened hinge- 

 plate. The pit is comparatively large and transverse ; the rest of 

 the hinge plate is feebly striate longitudinally. Along the upper edge- 

 of the posterior part of the hinge-line is a row of erect tubercles. 



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Left valve of Aviculopecten semicostatus, Portl., sp., showing the hinge-plate and: 

 interior view of valve. Lower Limestone series, Auchenmade, Ayrshhe. 



The name Aviculopecten was given by M'Coy to those Paleozoic 

 forms of Pecten in which the posterior ear was larger than the 

 anterior, and the supposed absence of a mesial ligamentary pit in 

 the hinge plate caused him to imagine that the affinity of the genus 

 was structurally nearer to Avicula than Pecten. We now know that 

 typical Pecten characters existed in Carboniferous times, and that 

 Aviculopecten, as at present restricted, is very closely allied to Pecten, 

 yet sufficiently distinct to merit a diiferent generic name. M'Coy 

 included Pterinopecten with Aviculopecten, but unfortunately we 

 know very little about its hinge plate, only that of P. rigidus 

 having been observed. There is no doubt of the propriety of Hall's 

 subdivision of the Palgeozoic Pectinidge, one which I have adopted 

 in my monograph on the British Carboniferous Lamellibranchs. 



