PARALLELODON CONCINNUS. 153 



there is a very marked difference between tbe two forms in surface-ornament ; 

 and for this reason I am unable to satisfy myself that the shells referred to 

 P. tenuistria by de Koninck belong to that species at all. 



In shape P. tenuistria closely resembles P. obtusus ; but the latter shell does 

 not possess radiating striae, and can at once be recognised if the test be preserved. 

 The approximation of the superior and inferior margins posteriorly in P. tenuistria 

 may be relied upon, when in the state of casts, in identifying the species. 



It is unfortunate that Messrs. Meek and Worthen gave the specific name 

 Macrodon tenuistriatus to a shell from the Upper Coal-measures of Illinois, which 

 has no affinity with M'Coy' s species, but seems to me to be probably identical with 

 P. semicostatus of that author. Tbe figure and minute description agree entirely 

 with some of the British specimens. 



Parallelodon concinncs, M'Coy, sp., 1844. Plate XII, figs. 16—22, 22 a. 



? Abca fimbbiata, M'Coy, 1844. Carb. Foss. Ireland, p. 71, pi. xii, fig. 8. 

 Modiola concinna, M'Coy, 1884. Ibid., p. 74, pi. xi, fig. 28. 



— — Bigshy, 1878. Thesaurus Devonico-Carboniferus, p. 309. 



— — Etheridge, 1885. Brit. Foss., pt. 1, p. 285. 



Specific Characters. — Shell transversely oblong, gibbose in front, obscurely 

 diamond-shaped. The anterior end is short, tumid, and the narrowest part of the 

 shell in a dorso-ventral direction. The anterior-superior angle is a right angle, 

 from which point the anterior border becomes semicircularly curved, and passes 

 gradually into the inferior border, which is almost straight, but sinuated anterior 

 to its middle point for the byssal opening; posteriorly it passes slightly upwards 

 to meet the posterior border at an acute angle. The posterior border is straight 

 and very obliquely truncate from above downwards and backwards ; above, it 

 makes a very obtuse angle with the hinge-line. This is much shorter than the 

 transverse diameter of the shell, straight and elevated posteriorly. The umbones 

 are obtuse, incurved, only very slightly raised above the hinge-line, pointed 

 inwards and forwards, and situated in the anterior fifth of the shell. They are 

 non-contiguous, and separated from each other by a well-marked ligamental area ; 

 comparatively broad in front, but becoming obsolete posteriorly. 



The valve is regularly swollen, but compressed towards the posterior-superior 

 angle, posterior to a line passing from the upper border of the umbo to the 

 posterior-inferior angle. The posterior end is lanceolate in outline, owing to the 

 acuteness of its lower angle. The byssal sinus is very well marked. 



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