This antique bottle is a nursing bottle. It would have been filled with milk and fitted with a rubber (or similar material) nipple and used to nurse infants. There is a very wide range of shapes and sizes for these types of bottles, but in general they have an ovoid pumpkin-seed-eqsue body with a flat side and a short neck. Earlier examples often have a pewter nipple.
Not too much is known about this particular variant. Searches online yield a 2000 response from Digger O’dell who noted that the item is a called a Turtle Nurser due to it’s shape. From Digger O’Dell: “It was originally fitted with a tube and nipple. The Seaside Nurser comes in both clear and aqua and the earliest ones date to 1878. In 1898, they were advertised as selling for 60 cents a dozen.” (Source)
Seen below is The Seaside Nurser and other freshly dug bottles on the day it was excavated (2011)