LEIORHYNCHUS LAURA 169 



tion of which has not been ascertained. The plications on these shells 

 are in general less prominent than in the figure of L. laura given by 

 Billings (1860), the young being entirely free from either plications or 

 fold and sinus. In the adult the number of plications is sometimes as 

 high as five on either side of the sinus in the pedicle valve, but is gen- 

 erally much less. The sinus bears a variable number of plications, four 

 being frequent, but as many as six faint and irregular ones have been 

 found. On the other hand, three or in rare cases two or one (?) occur. 

 New plications generally appear in the adult portions of the shell by 

 bifurcation of the existing ones, or sometimes by intercalation of new 

 ones. Those individuals in which a large number of plications exist 

 in the adult stage may be considered as more accelerated than those in 

 which the number remains smaller. The number of plications on the 

 fold is in general proportional to that in the sinus. In what appears to 

 be the least accelerated shell found, one strong plication occupies the 

 center of the sinus, and corresponding to it there is one deep depression 

 in the fold. On one side of this central plication a minor one occurs 

 and reaches about two-thirds to the beak. On the other side the corre- 

 sponding plication has just appeared near the front of the sinus. The 

 shell in this case is an adult. A somewhat more accelerated individual 

 has a strong central plication with a minor lateral plication on each 

 side, the lateral ones not reaching as far as the central one. 



Nicholson and Schuchert consider R. laura Billings and R. multlcostus 

 Hall synonymous, while Hall and Clarke * consider them distinct. 

 These authors figure a specimen of the pauciplicate form above described 

 as R. laura, but a reference to Billings' figure shows that he used a mul- 

 tiplicate form as the type of his species. This type occurs abundantly in 

 a crushed condition in the bituminous shales, intercalated in the Encrinal 

 limestone. The plications extend nearly to the beak, and, as far as can 

 be judged, the shell is proportionally broader than that of the lime- 

 stones. This variety is identical with the New York species, and appears 

 to be the one described by Billings, thus having priority over L.multicostus. 



The shell from the calcareous layers above described and figured by 

 Hall and Clarke as L. laura we believe to be identical with L. huronensis 

 Nicholson. f The figured individual is more elongate proportionally 

 than the majority of our specimens, while the width is the same. The 

 young of these shells are all proportionally wider than the adult, being 

 in one stage wider than high, while in the adult the proportion of width 

 to length is about 9 to 10 or even relatively narrower. The propor- 

 tional increase of width to length in adult and old age individuals is 



* Paleontology of New York, vol. 8. 



f Paleontology of Ontario, 1874, page 90, figure 28. 



