MARYLAND GEOLOGICAL SURVEY 731 



of the shell ; another faint groove, which is less conspicuous upon the larger 

 individuals, crosses the post-umbonal slope in a nearly vertical direction 

 from the lower margin of the deep groove already described behind the 

 beaks to the posterior extremity of the basal margin; surface of the 

 anterior half of the shell, as shown in impressions of the exterior, marked 

 by exceedingly fine, regular, concentric striae, parallel with the shell 

 margin, twenty or more of which occupy the space of 1 mm. These stria? 

 towards the antero-cardinal extremity are crossed by finer radiating stria?, 

 which produce an exceedingly fine reticulate pattern upon the shell sur- 

 face. Markings of the posterior half of the shell unknown. 



" Casts of the irregular burrows of this species are sometimes of com- 

 mon occurrence in the Merchantville clay, penetrating masses of fossil 

 wood, and on tracing these burrows to their termination casts of the shell 

 can usually be found, sometimes in excellent condition. Some masses of 

 the tubes are all much smaller than those in other masses, but all the tubes 

 in one group are usually of approximately the same dimensions. It was 

 at first thought possible that the different sized tubes indicated different 

 species, but the shells are all essentially the same, whether from large or 

 small tubes, in all masses observed in the Merchantville clay marl. A 

 mass of essentially identical tubes has been found in the Marshalltown 

 clay marl, however, associated with many individuals of Martesia bisulcaia, 

 which have a very different shell, described in this report as Turnus 

 kiimmeli. Other similar tubes occur sometimes in the Navesink marl, but 

 the accompanying shells have not been observed. These tubes, however, 

 seem to be straighter and they probably belong to another species. 



'' The type specimen of T. irregularis is without data as to locality or 

 horizon, and the description of the shell itself is too meager to be of any 

 use in identification. Inasmuch, however, as the Merchantville clay marl 

 is the horizon where burrows of this sort most frequently occur, and as 

 Gabb described numerous fossils from this horizon in Burlington County, 

 Xew Jersey, it is altogether probable that the type specimen is specifically 

 identical with the shell here described. 



" Morton evidently applied the name Teredo tibialis to all the Teredo- 

 like tubes he found in New Jersey, but the name is still retained for the 

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