90 CRETACEOUS PELECYPOUA 



The shells are in form intermediate between the Saxicavid/e and the SolesidjE ; 

 thej' are elongated, inequilateral, occasionally slightly nacreous inside, with the 

 l)allial impression sometimes partially interrupted, or undulating, and with tiic 

 pallial sinus deep. The hinge has numerous teeth in each valve ; there is a very thin 

 cartilage substance present, situated above the posterior laminar teeth, and besides 

 a strong ligament externally above it, partially covering the same and somewhat 

 internally attached behind it posteriorly. II. and A. Adams place this family in 

 the VENEEA CEA next to the Petricolid/e, which can scarcely be admitted as 

 correct. Tiie animals of both GhiKcoiwiiiija and Tanysiphou which I have examined 

 are, as I stated, externally much like those of IJija or Anatlna, while in the 

 Petricolidm we find two short sub-ccpial sii)hons separated for the greater part of 

 their length, and not covered \<\\\\ the C2)idermis of the shell, Avhich in the 

 Glauconomyidm forms a regular tube over the siphons. These are, however, fully 

 retractile within the shell, as vfc find them in the Solenid^, Mactridm, and some 

 species of the AxatinidjE, Avhile in other allied forms, like Analiua or Cyrtoduria, 

 the siphons are not perfectly retractile. 



The only resemblance existing between the Glauconomyidje and Petricolidm 

 lies in the dentition of the hinge, not in the structure of the shells, which are 

 thin and somewhat pearly in the former, thick and solid in the latter. The anterior 

 hinge-teeth are in both no doubt similar, but they do not, on the other hand, differ 

 essentially from the hinge-teeth of NovacuUna. In the JPetricolidm the single one 

 or two posterior teeth fit closely beside each other, exactly as do the anteriors when 

 the valves are closed ; but in the Glaucosomyidm, the posterior teeth are bifid, their 

 smaller portions fitting one above the other, and the hinder larger portions possess 

 flat surfaces for the attachment of a thin white cartilage. This last is very distinct 

 in Tanysiphon, in which the ligament is small, but in Glauconomya, in Avhicli the 

 cartilage becomes sometimes quite obsolete, the ligament jn'edominates. For this 

 reason I do not wish to lay too much stress upon the presence or want of a cartilage. 

 The greatest resemblance Avhich I can find, as regards the structure and general 

 character of the shell of Glauconomya, is that with Standella (of the lvteariin^e) ; 

 the cartilage processes are, of course, very different, but there actually is a perfect 

 analogy between both in their position with reference to the external ligament. 



In habitat tlie GLAUcoNOMYiDAivao^tlj resemble the Solenidm ; both bm-row 

 in sand and mud, and most of tlie species live in brackish water ; they appear to be 

 confined to the waters of the Eastern, especially the Indian seas. I have observed 

 several species of Glauconomya burrowed in sand between tide-marks; they seemed 

 to prefer particularly those localities where a small stream of fresh water enters 

 upon tlie beach. Thus comparing all these characters of the shells and the 

 animals, I do not think they are in favour of the classification of the Glauconomyid/b 

 in the VENEllA CEA, but that they clearly indicate a position near the Saxica rin^, 

 forming in many respects a ti'ansition from these to the SoLESiDyE. 



There are at present only two genera, Glauconomya and Tanysiphon, sufficiently 

 well known to be placed in this family, but it appears to me probable that this 



