TETRACLITA YITIATA. 341 



exposure of the tips of the upfilled parietal tubes. The lower part of 

 the surface is generally well preserved, and is formed by very irregular, 

 branching, longitudinal, slightly prominent, ridges. Radii of moderate 

 width, with their summits oblique. Orifice rather large, rounded 

 trigonal. 



Scuta, rather narrow, with the upper part acuminated : the external 

 surface generally much disintegrated, and marked with some irregular 

 blotches of dark red. Articular ridge not prominent ; articular fur- 

 row rather deep : adductor ridge, distinct from the articular ridge, 

 pretty w r ell developed, as are the crests for the rostral and lateral 

 depressor muscles. In young and w r ell preserved scuta, there is an 

 external, medial, hyaline band, corresponding with the hollow under the 

 adductor ridge, and caused by the thinness of the valve along this line. 

 Terga : externally, the carinal half is longitudinally and very feebly 

 striated. Internally, the articular furrow is very wide, but shallow, and 

 of unusual length, owing to the preservation of the upper part of the 

 valve ; the articular ridge is not prominent. The spur does not 

 actually join, as in all the foregoing species, the basi-scutal angle, 

 but is separated from it by a short piece of basal margin ; its two 

 sides are more nearly parallel than is usual, and the end is regu- 

 larly rounded. It is always rather narrow, though the width varies 

 considerably (fig. 3d, 3e). It extends in the same straight line with 

 the middle of the articular furrow. The terga, though not possessing 

 any striking characters, differ considerably in appearance from those 

 of the other species. 



Structure of the Shell and Radii. — The parietal tubes are remarkable 

 from their irregular shapes, and unequal sizes (fig. 3 b), — hardly two 

 resembling each other. Sometimes a single elongated tube will reach 

 across the whole thickness of the walls. The septa between the tubes are 

 rather thick and rugged. The tubes are generally darkly coloured from 

 the adhering corium ; they are solidly upfilled, but only in the upper- 

 most part, by dark chocolate-red shelly matter. The radii are formed 

 by irregularly branching ridges, with the interspaces filled up solidly. 

 The square edges of the alse are much thicker than in any other 

 species, and are furnished with transverse ridges, which are sometimes 

 slightly branched. The inner lamina of the walls near the basis, in 

 most of the specimens, is irregularly and longitudinally ribbed, the 

 ribs being longitudinally striated. The sheath and the upper part of 

 the inner lamina of the parietes are clouded with chocolate-red. 



The animal's Mouth and Cirri were ill-preserved ; but I was able to 

 make out that the labrum had some strong teeth, and that the man- 

 dibles were furnished with five teeth, a greater number than in any 

 other species. The palpi had parallel sides as usual. In the sixth 

 cirrus, the segments had four pairs of main spines, instead of the usual 

 number of three. 



Affinities. — This species does not appear to be particularly related to 

 any other one : perhaps it is rather nearer to the two following than 

 to the foregoing species. The irregular parietal tubes, thick-edged 

 alse, form of terga, five teeth to the mandibles, and four pairs of spines 

 to the segments of the posterior cirri, are its chief characteristics. 



