72 



ISCHNOCHITON. 



Median valves are moderately wide, nearly straight posteriorly, 

 the hinder ones with a slight median beak with distinct diagonal 

 furrows and ridges, dividing them into median and lateral areas ; 

 the median areas are covered, on the sides, with fine but very 

 distinct longitudinal grooves, with the intervening ridges narrow 

 and rounded, more or less confluent and broken up into granules, 

 near the diagonal lines towards the median ridge becoming finer and 

 irregular, and finely granulous anteriorly and along the carina. 

 The lateral areas are more elevated and covered with stronger 

 radiating ridges, broken up into oblong and rounded, flattened 

 granules, and separated by narrow radial furrows. The lateral 

 insertion -plates of the median valves project but little beyond the 

 upper lamina; they are sub-truncate, with a thin notch or slit cor- 

 responding to the diagonal line above. The posterior valve is 

 transversely elliptical, with the posterior edge evenly rounded ; the 

 front area is as in the preceding ones ; the posterior area is covered 

 with fine radial and concentric grooves, dividing it into radiating 

 rows of small rounded granules; the articulating plates of its front 

 edge are rather wide, broadly rounded or sub-truncate, and separated 

 by a broad, rounded median sinus ; posteriorly the inner surface is 

 marked by about sixteen radiating lines, terminating in thin 

 notches of the inserted edge, which is very narrow and simple. 



The marginal membrane is rather narrow and covered with 

 rather stout, prominent, oblong and obtuse spinules, regularly 

 arranged in quincunx, their ends looking like granules, at the 

 edge and on the lower side these are replaced by small slender 

 spinules. 



Head rounded; hood large, the sides produced backward into 

 rounded lobes. Foot long and rather narrow. Gills about twenty- 

 four on each side, extending from the posterior end of the foot to 

 about its anterior third. 



Length, 17 mm.; breadth, 8 mm.; height, 5.5 mm.; length of shell, 

 15.5 mm.; breadth, 7 mm.; length 1st valve, 3 mm.; breadth, 6 mm.; 

 length 3rd valve, exposed part, 2 mm.; breadth, 7 ram., length of 

 posterior valve, 3.5 mm.; breadth, 6 mm. (Verrill.') 



Norway; O/ Martha's Vineyard, 101-194 fms.; of Fernandina, 

 Fla. 



Lophyrus exaratus SARS, Moll. Keg. Arct. Norv. p. 113, t, 8, f. 

 1 ; t. II, f. 1, 1878. Trachydermon exaratus Sars, VERRILL, Amer. 

 Journ. Sci. xxiv, p. 365, 1882; Trans. Conn. Acad. Sci. vi, p. 208, 



